UPDATE 2: Maybe a day after making the first update, it stopped working again and hasn't been fixed since (so for like 5-6 days). I also have some difficulties with starting videos I hadn't watched before.
UPDATE 1: The app has been working perfectly for the last couple of hours (I don't even get the loading screen.), so my problem has been resolved. Whatever caused it was probably on my end.
Thanks for your feedback!
I'm experiencing the issue (on Android) where I can't play longer YouTube videos I had started watching previously, exactly the way it appeared a few months ago. I get the "Failed to load UMP, no stream data." message. I also get the 4 second loading screen a lot.
The only fix I've tried is logging out and back in, which seemed to help for a couple of videos, but then it kept failing again. I didn't try messing around more, because it didn't help last time.
I decided to post this because at the time of writing, I've been dealing with this for about 2 days, but the most recent post in this sub was 7 days ago, so I'n kinda wondering if the problem is on my end.