Cannot save file to vault

I have created a vault in the cloud and a *.kdbx file there. I can open it on my smartphone with keepass2android, but when I change it I get an error message when I save it. Does anyone have an explanation?

See here: Workflow with keypass2Android database file in vault

Ps: storing an already encrypted file (kdbx) in an cryptomator vault does not gain extra security to the encryption.