This is an odd one but my DK (only my DK) had had issues for months where anytime I do anything on him (mount, use abilities, etc) I get a message that "Another action is in progress". It seems to be cosmetic in that it doesn't prevent the action I'm attempting but it eventually got to the point where I individually enabled each of my ... 200ish ... add-ons to figure out which one was causing the message and it turned out to be ErrorMonster. Zero add-ons enabled, no message, enable ErrorMonster, messages.

I have about 30 toons that this ISN'T happening with so I can't explain it being just my main account DK. It may be some corrupted saved data (if EM saves anything per-toon), in which case let me know what I'd need to do to reset it and I'll give that a try.