Hi Scott,
I had a chat with an analyst yesterday (ESRI Incident #1202165). A complete uninstall and then download and install from the website worked for me. But this is not an ideal situation. Unfortunately, he decided to come to the conclusion that the problem is ours.
So here's more background. Yes, the problem arose from upgrading directly from V2.1 to V2.1.1 via the prompt in Excel. I am an administrator on my machine, but the problem was brought to my attention by employees who are not administrators. They were able to walk through the update process, but then were thwarted by a lack of an ability to sign in (as I mentioned previously). I will also note that PowerPoint was not affected by this issue, only Excel.
I'm now attempting to get the users that have not attempted to upgrade to uncheck the "Check for Updates" box in Excel. Our agency will be sticking with V2.1 for now. But there is some buggy behavior that should be resolved.
Heath