Hi @michellemayer,
That’s fine re: sharing the text - now that I can see this error, that gives me a better sense of what the issue might be, and I think it may be tied to this other issue: Pubmed XML plugin not escaping special characters correctly in OJS3 - which would mean that you may wish to try updating the OJS PubMed export plugin that contains the bug fix for this issue, and then re-try your export. Also in this post they point out a validator specifically for PubMed, which you may wish to try as well (I wasn’t aware of this).
For the abstract piece - do your posters have formal abstracts in their metadata like the articles do? It looked like they didn’t from the screenshots you shared in the earlier post. It should be fine to have an empty <Abstract/> tag - it just serves as a placeholder for articles that don’t have abstracts in the article metadata (e.g. editorials).
-Roger