hi,
That may be related to: howtoforge.com/forums/showthread.php?t=23650
I am also facing a problem. I install the RoundCube package into my ISPConfig but I can't make any kind of authentication there. I try old accounts, new accounts that I create, with e-mail, without mail, with ISPConfig user, without, everything and I don't know what's happen. Have you any idea about it?