I have been frequently getting this error message on Minecraft Bedrock Windows 10:
You are out of data storage space and Minecraft is unable to save your progress! Minecraft will return you to the Main Menu to clear up storage space.

I first noticed it while playing a downloaded Realm that I've played on this computer before, but this time when I started it only went a few seconds before crashing to that message.

However it's gotten worse. I had deleted all packs and worlds, reinstalled the game, started a couple of new worlds and it happens again! I've got less than 100 MB of save data at all, with hundreds of GB open!
Even weirder, it doesn't fully return to the Main Menu. If I click Settings, it brings up the settings menu from while playing the world that just crashed instead. Also if I try to load any save file in that state, the program fully crashes and closes.

This has made playing Minecraft very difficult, since at any moment it may just kick me out and then that world is unplayable for good.

The only way around it I've found where I can consistently play, so far, is if I join someone else's world, which isn't what I'm wanting to do.