Says "account already playing" when rejoin multiplayer world

Since an long time, when playing with my son on iPad,

I cannot rejoin the game the next day or anytime at all anymore at all. It always gives the message that my account is already playing, which is not correct. It works, if we open a new world. But I cannot join the old world. See attached picture.

Tried everything already. Also a complete reinstall. Without avail. Can you help?