Logging in to Horde webmail produces the "500 Internal Server Error" after a minute. Only specific mailboxes are affected by the problem.
This problem may be caused by the existence of a large amount of messages in the mailbox (30,000 or more), which causes the PHP process to reach the timeout limit before all messages are listed during the login process.
Open Internet Information Services (IIS) Manager >
<ServerName> > FastCGI Settings, select
%plesk_dir%\Plesk\Additional\PleskPHP5\php-cgi.exe, click on Edit..., and increase Activity Timeout to up to 300 seconds. Restart the webmail(horde) site.
This will allow a successful login to the Horde Webmail interface. To avoid the problem, it is recommended to delete old messages or move them to the IMAP subfolder.