I have a similar issue - in Chrome (but not Firefox). I've had a lengthy exchange with LP Support about it, but none of the suggestions (nuke everything and reinstall, new profile, etc.) fix the problem. Worse, I also have Duo Security enabled as a second factor and, in the past, I would always get the Duo prompt rather than Yubikey, unless there was no network. Not anymore. Oddly, when I first (re)install LP I get the Duo prompt, but after that it's always Yubikey.
This is new behavior within the past week.
At least I don't get the "access blocked" message.