I think if the day changes and you have an unfinished Union raid battle ongoing (in other words, it fails) then that party locks. Until the next reset.
I haven't had this problem, but that's what a some Union members have complained about.
So it's been a couple days, I've checked every quest available, and it's still saying I have a pending active quest or raid battle with nothing in the pending raid tab. My guess is it was either a daily or a raid that I cleared with a different party or never actually "accepted" - that is finished and just went to a different menu/as a result of using 2 different browsers (phone/computer/laptop all are used for this game). Any way to actually track or contact someone from dev team about this? I have tried selecting every quest with the "pending" party at 0 AP to no success so not really sure what to do. Not really a huge deal granted as there's so many parties, just annoying to have one forever locked for some unknown reason.
Another union member and I had the same problem. It's due to the union event. It probably ended with your team still in an unfinished one and glitched it. Send a support ticket. I reported the problem 2 days ago and they just fixed it for me today.
I'm honestly surprised you didn't message them after the first day it happened.