Hello Bozana,
my team found that the dtd they used was for OJS 2, therefore not compatible. Since, they created an xml using dtd for OJS 3 and it has imported a test article.
I still have some queries, which I have turned to them, regarding importing all metadata (discipline, keywords, accept date/publish date, author affiliation, etc.). Can I get back to you if they still have problems with these?
I have appreciated your reply and this feature is very important for a good use of OJS.
Thanks again,
Lucia