I recently was working on a deployment where this error was because there wasn't enough memory on the system.
If you think you have enough (1gb is generally recommended), try to increase the tomcat heap
Code:
su - zimbra
zmlocalconfig | grep mem
Look for the tomcat heap one. I can't remember exactly what it says, but it's something like "tomcat_heap_memory=30". Take that line and increase the number to something like 40 or 50 by running:
Code:
su - zimbra
zmlocalconfig -e (tomcat mem string)=50
tomcat restart