A client recently migrated from Plesk to CPanel using their migration script; the migration was done by the host but left many sites broken. They hired me to fix up his sites which were broken from the migration. Some things to notice to help you fix problems:

All of the sites are in /home in a directory that maps to the username.

The permissions of some files from the migration script may be screwed, you can fix them with commands like the following:
[bash]chmod +r *[/bash] For allowing reading all the files in the current directory.

Recompiling Apache from the control panel is probably a good idea.

It’s also a good idea to increase the maximum memory allowed to PHP from 32mb to something like 128mb or 256mb. You can configure this under the PHP settings in CPanel. Out of memory errors in PHP are specifically caused by this setting.

It also helps to look at the logs to see what may be going on.
Logs are located in:
[bash]
/usr/local/apache/domlogs/
[/bash] Further php error logs end up in the /home/username directory.

Hope this helps you solve your problems. If not, you could always hire us to debug your issues.