Hi @Oberdan,
The rest of the stack trace that appears in the log below that error message should help clarify where the userss plugin is being invoked from. It’s likely to be a malicious plugin. OJS 3.3.0-14 is quite out-of-date, and there are known attack vectors against it; this is a good opportunity to update to the latest 3.3.0-x release.
Regards,
Alec Smecher
Public Knowledge Project Team