I don't want to steer you wrong here and get your system into trouble. I can only tell you what worked for me for Fedora 14 64 bit. The steps are fundamentally the same.
I am not the least bit familiar with Ubuntu, but I did read Falko's tutorial and it is very much in line with Fedora's general steps. When I have problems that just don't seem to get repaired by file reconfiguration, etc. IF POSSIBLE, I like to do a clean uninstall, then reinstall. Here is what I saw with Ubuntu:
aptitude install squirrelmail
I saw the "aptitude" command numerous times for installs, leading me to understand that this is the equivalent of "yum" for Fedora. So to replicate, I had:
# yum remove squirrelmail
Next, I had # yum clean all
I then went through the process of reinstalling squirrelmail fresh. After the reinstall, I logged into ISPConfig, phpMyAdmin, and did a database repair on ISPConfig db and MySql db, then I:
Ran from root: # postfix flush
This worked for me, but please proceed with caution and maybe wait until Falko or Till comment on this. Again, I am not the least bit familiar with Ubuntu...only CentOS and Fedora.