I ran into this problem: I tried to view the list of recordings and nothing happened.
The error message from "tail /var/log/apache2/error.log" was the following,
[Mon Aug 23 11:53:09 2010] [error] [client 188.8.131.52] PHP Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 82 bytes) in /usr/share/mythtv/mythweb/modules/tv/recorded.
To increase the memory limit I used to edit php.ini, but it seams those are only the global values. For mythbuntu you will find the file needing editing here:
Search for the term php_value memory_limit and change it to something bigger. I use 1024M as I have lots of RAM and just don’t want to bother with it again.