When you try to access a 2.1 endpoint, you're presented with a '502 Bad Gateway' error.
So long 2.1, it's been nice knowing you!
This puts some pressure on me to update jPhotoFrame to use the 2.5 endpoints, which means using a different library and some code rewrite. I hope to do it this week.
By the way I looked but could not find an official announcement from OWM about discontinuation of the 2.1 version of their API. I guess they did it quietly without a care that existing projects will be broken.
Update: jPhoto frame is now using OWM 2.5 APIs.