I have this same problem. We are also using scalix, however, we have Avast! ADNM. The problem we see ( 100 users ) is that the INBOX cannot mark items as read, thus they stay bold. This is more than a minor inconvenience, since we cannot tell which emails we have responded to, and which are new. If I close outlook and reopen it, the problem will disappear for a random amount of time, but it always comes back.
I've been told on the scalix forums to disable Avast!'s outlook scanning, but I'm not sure that is such a good idea. We do use ClamAV on the MTA, but I'd still rather have the problem fixed, and have two AV email scanners than just the one.
I've also been told this is a problem /w NOD32 as well.
Any advice?