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[22-Mar-2017 17:37:27 +0100]: <epf86f94> IMAP Error: Login failed for test@domain.com from x.x.x.x.x. AUTHENTICATE PLAIN: Authentication failed. in /usr/share/roundcube/program/lib/Roundcube/rcube_imap.php on line 198 (POST /webmail/?_task=login?_task=login&_action=login)
Just checking. It should be properly installed. I notice that roundcube-core has several suggested extras in the dependency list. I wonder if any might help.
I entered Authentication failed. in /usr/share/roundcube/program/lib/Roundcube/rcube_imap.php on line (POST /webmail/?_task=login?_task=login&_action=login) in DuckDuckGo and found lots of hits with some marked SOLVED. (note that I dropped the line number from your error message) One of them might help you track down your problem.