I tried swapping to the global server. All went well except for one thing - most of the time when I go to study a schedule which has words to teach me, it fails to load and instead just shows me a dancing カオちゃん forever. There's two loading screens as part of this process: the first one to load the quiz itself, and the second to load the "new items" introductory dialog. It's the second one that fails.
I swapped back to the default server a couple of days ago to make sure that was the cause of the issue, and I haven't seen it since. I still have "faster assets" enabled, so that doesn't seem to be a factor.
I'd be surprised if this is still an issue - this definitely was tied to faster assets. I recently updated the experimental page to note that now, even if it says "on", the setting has been disabled globally, as the issue you are describing was being experienced by everyone using the setting.
There is a way to fix it, but it is much more involved than I'd like to get at the moment (when the previous system is working fairly well already.)
Aha, it does seem to be fixed now even when I swap back to the global server... My test timing must have coincided with your fix. I don't think I could ask for much better service than a fix that lands before I even submitted the report :)
