This issue has an updated article.
We had looked into issues with the ISP (there were several misconfigurations in the setup for the Comcast Business connection of theirs), issues with DSL modems, issues with IE6/IE7/IE8, etc.
Here's what I did that seems to have fixed things:
1) The time was off on the server by 15 minutes. This was fixed and the ntp configuration was fixed so that the problem shouldn't occur again.
2) The timezone was wrong in Zimbra. This was the open source version of Zimbra and I had upgraded over a series of versions, jumping from an early release through several major releases. At some point, the timezone (tz) became an invalid option. I fixed the server's time zone (and thus the corresponding domains) by setting it to US/Eastern.
3) There were a few other options that I changed that had been added. I wish I had written them down, but I didn't think what I was doing was going to fix things, only that the settings weren't what I expected/wanted them to be. For instance, I added all of the zimlets at the server level as they were disabled there.
I saved everything and then went on to other things. I received an email with 24 hours from the user reporting that everything was, once again, working.
There are a couple of possiblities for what fixed things and it may have been as simple as a restart (though I'd tried that before). The two biggest things are item 1 and 2, where the time was fixed and the timezone was fixed, though some of the other fixes I did may have been a factor.
My advice, if you are having this problem, is to go through the settings on the server and make sure everything is set as expected EVERYTIME after you do an upgrade. In my case, the default settings weren't right and new options appeared.
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