Doesn't ring a bell at all -- the NPE is in a strange place, not exactly sure what would cause it to happen. Have you done anything strange to the account, like tried to disable the index from config or something like that? Have you tried restarting the service to see if the error goes away?
Next step is to turn on index debugging and see what search is doing when the NPE happens:
Open up /opt/zimbra/conf/log4j.properties and add the following lines to the END of the file:
Restart your server, or wait 5 minutes for the change to take -- then repro the problem. Cut out the time period with the repro from your mailbox.log and post it here or PM it to me.
If your server is very busy, it will be easier to reproduce this during an off time, to lessen the noise in the log.