Problem importing issue and article PDFs in OJS 3.1.2 (XML)

I want to add a completely new issue including separate PDFs for individual articles with the XML file below. It is kind of mashed to gather from different examples, maybe I am trying to do something that is not possible?

<?xml version="1.0" encoding="UTF-8"?>
<issue xmlns="http://pkp.sfu.ca" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" published="1" current="1" access_status="1">
    <issue_identification>
        <volume>2</volume>
    </issue_identification>
    <sections>
        <section ref="ART" seq="0" editor_restricted="0" meta_indexed="1" meta_reviewed="1" abstracts_not_required="0" hide_title="0" hide_author="0" abstract_word_count="0">
            <abbrev locale="de_DE">ART</abbrev>
            <title locale="de_DE">Artikel</title>
        </section>
    </sections>
    <issue_galleys>
        <issue_galley>
            <label>Issue PDF</label>
            <issue_file>
                <file_name>issue.pdf</file_name>
                <file_type>PDF</file_type>
                <file_size>1110854</file_size>
                <content_type>1</content_type>
                <original_file_name>issue.pdf</original_file_name>
                <date_uploaded>2021-10-28</date_uploaded>
                <date_modified>2021-10-28</date_modified>
                <embed encoding="base64">(..)</embed>
            </issue_file>
        </issue_galley>
    </issue_galleys>
    <articles>
        <article stage="production" locale="de_DE" section_ref="ART" date_submitted="2021-10-28" date_published="2021-10-28">
            <id type="other::zenon">001364448</id>
            <title locale="de_DE">Joannes Grammatikos (Philioponos) von Alexandrien und die arabische Medizin </title>
            <abstract locale="de_DE"></abstract>
            <copyrightHolder locale="de_DE">Digitale Ausgaben von Monographien, Reihen- und Sammelwerken des DAI</copyrightHolder>
            <copyrightHolder locale="en_US">Digital editions of monographs and series of the DAI</copyrightHolder>
            <authors>
                <author primary_contact="true" include_in_browse="true" user_group_ref="Author">
                    <givenname>Max</givenname>
                    <familyname>Meyerhof</familyname>
                    <email></email>
                </author>
            </authors>

            <submission_file stage="production_ready" id="1">
                <revision number="1" genre="Artikeltext" filename="article-0.pdf" viewable="false" filesize="224864" filetype="application/pdf">
                    <name locale="de_DE">article-0.pdf</name>
                    <embed encoding="base64">(..)</embed>
                </revision>
            </submission_file>
            <article_galley approved="true">
                <name locale="de_DE">PDF</name>
                <seq>1</seq>
                <submission_file_ref id="1" revision="1" />
            </article_galley>
        </article>
        <article stage="production" locale="de_DE" section_ref="ART" date_submitted="2021-10-28" date_published="2021-10-28">
            <id type="other::zenon">001364451</id>
            <title locale="de_DE">Koptische Grabstelen : Ihre zeitliche und örtliche Einordnung </title>
            <abstract locale="de_DE"></abstract>
            <copyrightHolder locale="de_DE">Digitale Ausgaben von Monographien, Reihen- und Sammelwerken des DAI</copyrightHolder>
            <copyrightHolder locale="en_US">Digital editions of monographs and series of the DAI</copyrightHolder>
            <authors>
                <author primary_contact="true" include_in_browse="true" user_group_ref="Author">
                    <givenname>Dora</givenname>
                    <familyname>Zuntz</familyname>
                    <email></email>
                </author>
            </authors>

            <submission_file stage="production_ready" id="1">
                <revision number="1" genre="Artikeltext" filename="article-1.pdf" viewable="false" filesize="232288" filetype="application/pdf">
                    <name locale="de_DE">article-1.pdf</name>
                    <embed encoding="base64">(..)</embed>
                </revision>
            </submission_file>
            <article_galley approved="true">
                <name locale="de_DE">PDF</name>
                <seq>2</seq>
                <submission_file_ref id="1" revision="1" />
            </article_galley>
        </article>
    </articles>
</issue>

I omitted the encoded PDF. When I try to import this file, I get stuck with an empty “results” tab in the Natives XML-Plugin View. A PHP error occurs:

[php7:notice] (...) ojs2: DB Error: Incorrect double value: '' for column `ojs`.`published_submissions`.`seq` at row 1, referer: (...)index.php/mdaik/management/importexport/plugin/NativeImportExportPlugin

Application Version:


Version history
Version 	Major 	Minor 	Revision 	Build 	Date installed
3.1.2.4 	3 	1 	2 	4 	2021-10-05
Server Information
Setting Name 	Setting Value
OS platform 	Linux
PHP version 	7.3.31
Apache version 	Apache/2.4.48 (Debian)
Database driver 	mysqli
Database server version 	10.6.4-MariaDB-1:10.6.4+maria~focal 

I created a follow up thread: OJS 3.1.2 XML import article embeds not processed

Update: I managed to get rid of the error by setting different ‘seq’ values:

<?xml version="1.0" encoding="UTF-8"?>
<issue xmlns="http://pkp.sfu.ca" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" published="1" current="1" access_status="1">
    <issue_identification>
        <volume>2</volume>
    </issue_identification>
    <sections>
        <section ref="ART" seq="0" editor_restricted="0" meta_indexed="1" meta_reviewed="1" abstracts_not_required="0" hide_title="0" hide_author="0" abstract_word_count="0">
            <abbrev locale="de_DE">ART</abbrev>
            <title locale="de_DE">Artikel</title>
        </section>
    </sections>
    <issue_galleys>
        <issue_galley>
            <label>Issue PDF</label>
            <issue_file>
                <file_name>issue.pdf</file_name>
                <file_type>PDF</file_type>
                <file_size>1110854</file_size>
                <content_type>1</content_type>
                <original_file_name>issue.pdf</original_file_name>
                <date_uploaded>2021-10-28</date_uploaded>
                <date_modified>2021-10-28</date_modified>
                <embed encoding="base64">(..)</embed>
            </issue_file>
        </issue_galley>
    </issue_galleys>
    <articles>
        <article stage="production" locale="de_DE" access_status="2" seq="1" section_ref="ART" date_submitted="2021-10-28" date_published="2021-10-28">
            <id type="other::zenon">001364448</id>
            <title locale="de_DE">Joannes Grammatikos (Philioponos) von Alexandrien und die arabische Medizin </title>
            <abstract locale="de_DE"></abstract>
            <copyrightHolder locale="de_DE">Digitale Ausgaben von Monographien, Reihen- und Sammelwerken des DAI</copyrightHolder>
            <copyrightHolder locale="en_US">Digital editions of monographs and series of the DAI</copyrightHolder>
            <authors>
                <author primary_contact="true" include_in_browse="true" user_group_ref="Author">
                    <givenname>Max</givenname>
                    <familyname>Meyerhof</familyname>
                    <email></email>
                </author>
            </authors>

            
            <submission_file id="1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" stage="production_ready" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
                <revision number="1" genre="Artikeltext" filename="article-0.pdf" viewable="false" filesize="224864" filetype="application/pdf">
                <name locale="de_DE">article-0.pdf</name>
                <embed encoding="base64">(..)</embed>
                </revision>
            </submission_file>
            <article_galley xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" approved="false" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
                <name locale="de_DE">PDF</name>
                <seq>1</seq>
                <submission_file_ref id="1" revision="1"/>
            </article_galley>
        </article>
        <article stage="production" locale="de_DE" access_status="2" seq="1" section_ref="ART" date_submitted="2021-10-28" date_published="2021-10-28">
            <id type="other::zenon">001364451</id>
            <title locale="de_DE">Koptische Grabstelen : Ihre zeitliche und örtliche Einordnung </title>
            <abstract locale="de_DE"></abstract>
            <copyrightHolder locale="de_DE">Digitale Ausgaben von Monographien, Reihen- und Sammelwerken des DAI</copyrightHolder>
            <copyrightHolder locale="en_US">Digital editions of monographs and series of the DAI</copyrightHolder>
            <authors>
                <author primary_contact="true" include_in_browse="true" user_group_ref="Author">
                    <givenname>Dora</givenname>
                    <familyname>Zuntz</familyname>
                    <email></email>
                </author>
            </authors>

            
            <submission_file id="2" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" stage="production_ready" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
                <revision number="1" genre="Artikeltext" filename="article-1.pdf" viewable="false" filesize="232288" filetype="application/pdf">
                <name locale="de_DE">article-1.pdf</name>
                <embed encoding="base64">(..)</embed>
                </revision>
            </submission_file>
            <article_galley xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" approved="false" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
                <name locale="de_DE">PDF</name>
                <seq>2</seq>
                <submission_file_ref id="2" revision="1"/>
            </article_galley>
        </article>
    </articles>
</issue>