Weird coincidence that the exact reason for this thread is addressed in https://github.com/nextcloud/android/releases/tag/rc-3.35.0-01. RC2 was just released yesterday.

key ring

I had a similar problem a long time ago on a new laptop and this is what it was for me. I didn’t have a keyring provider that the nextcloud client could use, so it would get a token when I entered the password, but couldn’t store it so it would be only in memory and vanish every time I disconnected.