taint.log

Noticed I would get a few messages about an Addon encountering errors. Started the log process and reperformed the actions (Attacking a combat dummy) to find it was Omnibar. I am not sure yet if this is the addon responsible for some other UI issues in combat Sometimes an issue will occur that will result in the CDs of all abilities on the action bar to no longer show their proper CD or to be greyed out. I suspect Omnibar but this did not occur during the errors in the attached log.

So I was able to get the action bar error and still only omnibar is noted. I do have CD #s turned on in the blizzard options and they are not present there so could be a blizzard issue.
taint.log

FWIW -- my cast bar also teleports from its default position all the way down to the bottom of the screen (still centered) after the UI goes into this state. I don't know if that is consistent for you.

Separately, I have noticed that certain classes being present in my arena matches seem to trigger it (might just be correlation instead of causation). I'm not sure which ones yet -- but I'll go several games without issues and then immediately hit the error within seconds of a solo shuffle round and every round for that whole match.

One match that I have in my details logs was: unholy dk / ret paladin / surv hunter / destro lock / disc priest / and me as holy paladin. DK had a controllable pet (in addition to uncontrollable abom / army etc ) / surv hunter had a controllable pet / warlock had a controllable pet (in addition to uncontrollable infernal).

I saw that your logs mention a pet bar -- which shouldn't be applicable for your monk or for my paladin right?

Happy to upload logs here if you give instructions how to capture them. They might even be on my machine already