Reviewer status unclear - stage ID undefined error

A reviewer agreed to review a submission. She got the automated confirmation email from the system. When I look at the submission as editor, the reviewer looks like she has not answered the invitation yet. The overdue warning is displayed and her agreement is not logged in the history. When I log in to her account, I see that she has access to the full text. From her view, the agreement to review is working, from the editor’s end, its not.

In the error log, I found the following related to this submission:

AH01071: Got error 'PHP message: PHP Notice:  Undefined index: stageId in /var/www/...ojsname.../lib/pkp/controllers/grid/eventLog/SubmissionEventLogGridHandler.inc.php on line 111', referer: https://...ojsname.../index.php/journalname/workflow/index/16858/3

AH01071: Got error 'PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/...ojsname.../lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/...ojsname.../lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/...ojsname.../lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/...ojsname.../lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'de_DE' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133PHP message: PHP Warning:  Illegal string offset 'en_US' in /var/www/ojs.openagrar.de/lib/pkp/classes/core/DataObject.inc.php on line 133', referer: https://...ojsname.../index.php/journalname/submissions

The journal is configured in German and English. How do I get this reviewer out of the ‘stage limbo’?
This is happening in OJS 3.3.0.8 and to my knowledge, it is the first time this was observed in our installation.