Had the same problem. In my case it was related to a permissions error.
Saw this in the logs:
[Mon Nov 14 10:24:56 2011] [error] [client 192.168.1.81] PHP Warning: include() [<a href='function.include'>function.include</a>]: Failed opening 'cache/modules//Uservardefs.php' for inclusion (include_path='[...]' in /home/sugarCRM/www/include/SugarObjects/VardefManager.php on line 108
I fixed it by giving write permission to user apache (in CentOS).
Code:
[root@crm2 www]# chown apache * -R
[root@crm2 www]# chmod 775 * -R
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