I'm having the same problem on a fresh Debian 5.0.3 64bit server, with ISPConfig 220.127.116.11 installed as "slave" server.
Here is an example of the error I get, trying to add a client:
failed to open stream: Too many open files in /usr/local/ispconfig/interface/lib/classes/listform_actions.inc.php on line 143
Some more info:
/home/admin# lsof |wc -l
/home/admin# ps aux | grep php
root 7593 0.0 0.0 5160 784 pts/0 S+ 23:02 0:00 grep php
I get the same "Too many open files in ..." error randomly across other sections.
Also, any website creation done from the master panel to the slave server remains forever in the monitor jobqueue of the master.... maybe it's related.
The master server works correctly on itself.