Yep, the updated firmware was the issue, just downgrade the previous one and it will work right away.

I upgraded my AP this morning and then began experiencing chromecast issues and stumbled upon this thread, after doing some digging I believe the problem lies within the updated firmware | '(3)...

The new firmware is what causes this to break, I updated mine this morning and then all of a sudden my chromecast stopped working, I checked the release notes and I believe this to be the cause "(3)...