If I copy the Squirrelmail to /var/www instead of using a symlink, I get these errors in the Apache logs, instead:
* SecurityException in Application.cpp:440: Handler not found in configuration
* Caused by KeyNotFoundException in Configuration.cpp:234: Handler "application/x-httpd-php" not found
If you remove and delete Squirrelmail files manualy, and install normaly as you shoud, this will work, dont do crazy stuff and ask help from there.
The better way is allways the clean and normal way.
this "way" works even when updating.
Ubunto is for workstations and normal Debian is for servers maybe is that your first problem.