I faced the same problem. The cause was a furniture add-on from the Marketplace. In my case, it was "Blocks Updated." However, I had previously installed other furniture add-ons, all of which caused problems.
The solution was a bit more complicated. Here's what happened:
I had to download a backup on the Realm. This wasn't possible because one of the add-ons was no longer compatible. The Realm kept returning "unknown errors" to the download.
Therefore, I had to deactivate all add-ons at once (and that was the only thing that worked). I then went back to the main menu and waited a minute.
Finally, I was able to download a backup under "Manage Realm." After reactivating all add-ons except the furniture add-on, our child (with a child account) was able to join the Realm again without any problems from their Android tablet.
I had to deactivate all add-ons EXCEPT the furniture add-on.
Conclusion: Add-ons are nice and great, but avoid those that promise additional furniture. They often don't work or only work inadequately.