Been using ISPConfig for quite awhile now and its very good - stable, well maintained, free, well you get my drift.
One thing however has bugged me since I've started using Horde as a Groupware. That I cannot get the Tasks to work properly.
I've previously asked for help here:
I've also asked for help on the Horde mailing list, but I hit a wall:
Any other logs, or what not please do not hesitate to PM or put it back here... I'm positive that I'm not the only one who has ISPConfig 3 and Horde running.