An update, still not solved this issue!
I found that the PDF:s of these articles was not downloadable from the frontend.
However, the following MySQL-command solved the PDF issue:
UPDATE files SET path = Replace(path, '//', '/public/');`
Now PDF:s can be downloaded on the frontend, but it did not solve the issue in the submissions. Is it something with the file stage number? In the error above the file stage number is “1”. If I change this number, what to change it to?
Is it connected to this issue?