Hi @toma,
Can you verify which PDF display plugin you mean?
For 500 errors it is ideal to see error output from the PHP error logs to see relevant error log messages from there. I know you said you don’t have access, but if you’re able to get the messages from a sys admin or someone else, that will be more helpful for troubleshooting.
There’s also this issue here: PDF files are not opening / downloading after server migration
Which was resolved by adjusting the files_dir config - something perhaps also worth looking at.
-Roger
PKP Team