Ryan,
Well.. figured out the issue. Can't say I have a clue about the 'why', but I know the 'how' now, and that's enough for us to investigate further and for you to hopefully get going again.
The issue was in the module "Search" setting for "OpenSearch autodiscovery" which was set to "Yes". I'm not sure what this does other than adding that extra link tag I referenced above, but obviously, there's something going on there. I set it to the "No" setting, and I was able to login and remain on the current page without issue. I also set it to "No" in 2 other Search modules.. not sure if they're used.
We'll definitely be investigating this for the 4.1.1 fix.. there were some changes in how we do redirection, as I mentioned, and either we'll work around this or revert to the old way if necessary.
Thanks for your patience, and as always, test, test, test!
Alex