Thanks for your answer. You are right, indeed it seems to be an intermitent problem: I tried again and now it works … Regarding log in, I am neither sure it is relevant, but I mentioned it because I first tried without being logged in, and in that case all seemed to work fine (maybe just by chance?)
I pushed two fixes. First of all a bug in forwarding the identity for oauth2 (Google, etc) identified users caused saving for only me to fail. It only worked for users with a local account (not available on the public instance). Second, the visibility rules have changed a little. You can (of course) always see your own files, but if users are not logged in your own is defined very loosely, eventually accepting the same IP. That is still the case, but if the file is supposed to be private it now demands the user is logged on so we can really identify the proper person.
Fixes pushed to the repo and the public swish instance is updated. Please test.