I am running Zimbra 8.0.5 on Ubuntu 12.04 LTS amd64 with about 150 active users. Periodically, Zimbra will go through a window of time when users cannot login to the frontend web UI (either the AJAX or standard version). They can authenticate successfully, but then they get stuck for minutes on the "Loading..." screen. System load and available RAM do not seem atypically high during this time, although iowait has jumped up to around 30%. Users who are already logged in are able to use the web interface without problems while this is happening. Which logs should I review to help pinpoint the source of this problem, and what should I look for in particular?