I have this problem with all the mailing lists on my CPANEL/WHM Server, when a user tryies to edit the list members. Does anybody knows something about it?

Bug in Mailman version 2.0.6

We're sorry, we hit a bug!
Please inform the webmaster for this site of this problem. Printing of traceback and other system information has been explicitly inhibited, but the webmaster can find this information in the Mailman error logs.

We have ran across this on a few occasions - it was due to a full partition. Can't recall which one. you might try the "df" comand to check if you have any filling up.
Hope this helps.