Same problem here.
OS: Debian etch
ii libsasl2 2.1.22.dfsg1-8 Authentication abstraction library
ii libsasl2-2 2.1.22.dfsg1-8 Authentication abstraction library
ii apache2 2.2.3-4+etch6
ii php5-imap 5.2.0-8+etch13
I've install about of 6 isp config with roundcube and all have the same problem.
For example if the password is 12345678 and I put 123456789 or 1234567 (yes, with 1 missin carcter) the webmails login ok..
It's a serius bug, with other webmails like talent this problem down not happen
Is there any feedback..
I hope so because I really like this webmail but I habe to use talent for every ispconfig installation.
Thanks!:(