PDFs on OJS don't open anymore


We are still using OJS and don’t really have an option for upgrade.

Two weeks ago something with our OJS must have happened, because now the PDFs are not opening on the site or in the system itself.

On web browser the message is this:
PDF.js r1.5.188 (graditev: 0e2d50f)
Message: stream must have data

In the system
500 Internal Server Error

Can you help me with the problem?

When I delete the old galley and upload the new one then it works.

I would really like to avoid uploading pdfs for ten years though and would really appreciate the help on this issue.