I use a Dropbox directory to distribute my KeePass files to different devices. The Dropbox directory is encrypted with Cryptomator. So far (for over two years) this worked flawlessly. Since a few releases, it is suddenly no longer possible to save a Keepass file after modification.
This is obviously an error caused by Cryptomator: as a test case, I used Keepass to access an unencrypted Dropbox directory: this is where read and write accesses work without error. So it´s neither a Dropbox nor a Keepass issue - and makes my life uncomfortable 
Apart from the fact that there is no added value in storing an already encrypted keepass file in a Cryptomator vault, this might help you.
I just received this helpfull advise from the Keepass community:
Uncheck Use file transactions for writing databases in
Tools>Options>Advanced(tab)>File Input/Output Connections