When i start Drwebd on my machine, it gives the following error;

Loading /var/drweb/bases/drw43204.vdb - Ok, virus records: 607
Loading /var/drweb/bases/drw43203.vdb - Ok, virus records: 409
Loading /var/drweb/bases/drw43202.vdb - Ok, virus records: 543
Total virus records: 45804
Key file not found!
Plesk authorization failed: HTTP request error [7]
Error: Plesk Software not running.
[FAILED]

Now drweb worked fine for the previous half year, but now fails all of a sudden. Anyone got an idea?

Server is a CentOS 3.6 machine with Plesk 7.5.2, fully updated. The firewall rules are failrly strikt (almost everything is closed with the firewall module), could that have something to do with it?