This roundcube error happens after an update on a plesk panel
Error accessing roundcube after upgrade ERROR- Plugin Manager Center - Branch: http://dev.myroundcube.com (Roundcube v1.0.5) Misconfiguration: Unregister markasjunk2 in ./config/main.inc.php. You can't register a plugin in main.inc.php which is configured to be loaded by Plugin Manager.
Fix:
Reinstall roundcube in the plesk panel under Update and upgrades