Sorry for the spam, I investigated a little bit. The problem actually only happens when I try to open a vault created by someone else OR created by myself and then opened and closed by someone else. In both cases, when I check in Google Drive online version, the “Owner” of the masterkey.cryptomator.bkup will be (or will be changed to be) someone else. If I copy the file/rename it and delete the original one then I’ll be the Owner of masterkey.cryptomator.bkup and then I’m able to open the vault again.
[actually, simply deleting the .bkup file has the same result]