What setting do you have set for spamassassin?
Also, have you updated spamassassin's checks using sa-update?
Currently I'm rejecting ~2500 messages using a few custom RBLs, and about ~1500 messages using spamassassin. I'd say something could be wrong with your setup.
From when I used webmin you need to have spamassassin installed in the default location, whereas ISPConfig has it installed within the /home/ directory.