Could you try reproducing the error in firefox and in the error console clicking on the like to the source code? It should link you to the line in the source code.
Does it look like this?
Code:
if(!resp && !this._currentRequest.cancelled) { That is line 4906 of the Admin_all.
js script, the line right after that throws an excepting which might relate to the error you got of an exception being thrown but not caught.
Also is there any further information in /var/log/zimbra.log or /opt/zimbra/log/mailbox.log?