I’ve seen similar issue reported for other XML exports (native, crossref, etc.) on OJS 2 and thought it’s worth to point out the issue with PubMed XML plugin. We are running OJS 220.127.116.11
PubMed XML export of an issue resulted in a file with “unterminated entity reference” errors.
Upon digging it was traced back to the journal title having “&” in the masthead. We replaced it with “and” for a quick fix, but since it’s not uncommon for journals to use “&” in the title, this may come up again.
P.S. PubMed XML validator is helpful for troubleshooting if you are running into other kinds of errors Citation file validator - PubMed Data Management - NCBI