this is the only error that I'm seeing in the mailbox.log file around that time. It happened to me for the first time yesterday, then again this morning. I didn't think anything much of it until my other user who is testing ZCO as well let me know that he had the same issue at the exact same time. As far as I know, nothing is going on with the server at that time (specific jobs).
Code:
2007-10-12 08:47:22,273 INFO [http-4443-Processor49] [] ZimbraSoapContext - requested session no longer exists: 10049
2007-10-12 08:56:51,538 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_amzn
2007-10-12 08:56:51,548 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_tracking
2007-10-12 08:56:51,549 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_travelagent
2007-10-12 08:56:51,553 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_wikipedia
2007-10-12 08:56:51,554 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_xslt
2007-10-12 08:56:51,555 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - Zimlet not found: /opt/zimbra/tomcat/webapps/service/zimlet/com_zimbra_ymaps
2007-10-12 08:56:51,559 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_tracking
2007-10-12 08:56:51,565 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_xslt
2007-10-12 08:56:51,567 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_amzn
2007-10-12 08:56:51,567 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_ymaps
2007-10-12 08:56:51,567 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_wikipedia
2007-10-12 08:56:51,572 INFO [http-4443-Processor49] [name=user1@foobar.com;mid=40;ip=192.168.10.81;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_travelagent
2007-10-12 08:56:43,169 INFO [http-4443-Processor37] [] ZimbraSoapContext - requested session no longer exists: 10049
2007-10-12 08:57:56,539 INFO [http-4443-Processor37] [name=user2@foobar.com;mid=2;ip=71.XX.XX.XX;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_tracking
2007-10-12 08:57:56,542 INFO [http-4443-Processor37] [name=user2@foobar.com;mid=2;ip=71.XX.XX.XX;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_xslt
2007-10-12 08:57:56,544 INFO [http-4443-Processor37] [name=user2@foobar.com;mid=2;ip=71.XX.XX.XX;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_amzn
2007-10-12 08:57:56,544 INFO [http-4443-Processor37] [name=user2@foobar.com;mid=2;ip=71.XX.XX.XX;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_ymaps
2007-10-12 08:57:56,544 INFO [http-4443-Processor37] [name=user2@foobar.com;mid=2;ip=71.XX.XX.XX;ua=ZimbraConnectorForOutlook/5.0.306;] zimlet - cannot find zimlet com_zimbra_wikipedia
2007-10-12 08:57:58,487 INFO [IndexWritersSweeperThread] [] MailboxIndex - open index writers sweep: before=3, closed=0, after=3 (0ms)
This looks like errors from the web client code, but I haven't logged in to the web client in a week. My tester is user1, and I'm user2. User1 was connected to the local network at the time, and I was connected from home.