I notice on two different stations running Airtime, one running 2.5.1a the other running 2.5.2 from git, that some users get endless "Processing...." instead of the library and/or now playing, that never resolves, and they can't see the library or now playing.
On the 2.5.1a installation, the problem follows their user login, which means it does not matter which browser or computer they are using, they only see "Processing..." and nothing else. I've been able to duplicate the problem by logging in as them from any machine, then if I log back in as myself, problem gone. Looked in the Chrome/Firefox consoles and don't see any error messages. Also the Airtime logs show no errors.
The only way to eliminate the problem is to wipe out the user's account completely and recreate it. That fixes it every time, but it's kind of annoying.
On 2.5.2, I found the problem only happens with certain browsers and systems (i.e. Chrome on a Raspberry, but not Chrome on a linux desktop), and it does not follow the user's login: I can be logged in on two machines simultaneously, one showing Processing... and the other showing the library and now playing just fine.