Which Ubuntu version do you use that causes this problem? I installed just a fresh Ubuntu 9.10 server with all updates applied and ISPConfig 18.104.22.168 and the mail delivery works, no maildrop errors. Maybe there is a bug in the maildrop package that occurs only when its is updated and not when its newly installed.
[update] This is the version of the maildrop package installed on my test system:
dpkg -l | grep maildrop
ii maildrop 2.0.4-3+lenny3build0.9.10.1 mail delivery agent with filtering abilities