If you experience a PR_END_OF_FILE_ERROR in firefox, I experienced that it might be caused by a fail2ban block for the given user IP on SSL / https pages.
Just as a note to myself if I should ever run into that problem again.
Further information can be found here for example:
No local fixes needed in my case - just check if the IP is in the blocked IPs table and remove it, if unwanted.
By the way: In Plesk the WAF (Web Application Firewall) may lead to this block, even if set to "Detection only"!
The log entires are scanned by fail2ban and may lead to jail entires!