ugh, i can attach only one picture, so this is S3 provider backblaze on the picture, S3 bucket that I created in it.
Joplin returns two errors "Malformed key", which definately is not because I copy pasted it, and if I try to sync everything from local to S3, I get famous "Unknown error"
I'm sure it will be in the log.
Just tested syncing with Cubbit S3 and it worked on Android, Linux, and macOS.
So in general it does work with alternative S3 providers.
On my iPhone the app crashes when trying to sync. This means the app crashes now a few seconds after opening. When offline, the app doesn't crash, only after enabling Wifi.