There are cases where you load into Webmail and the message "Server Error (OK)" appears, though your emails do not load.
The most common reason for this would be that the Dovecot Index Cache for the mailbox in question becomes corrupted. We have some information on that here: Why can I see my email files from File Manager, but Webmail does not show them?
If this is not the case; The issue may be that the mailbox has individually corrupted files. To address the corrupted files, please see below.
1. Access your server through SSH.
2. Tail the Dovecot log with:
tail -f /var/log/maillog
3. Log into Webmail, and watch the log file for messages similar to:
Error: Corrupted record in index cache file
The error message will provide you with the path of the corrupted message. Remove the file from the mailbox, or move it to another directory for temporary. Repeat this process until webmail loads.