thank you for your reply.
In fact, as soon as I change the extension, the project is imported to the BIMcloud. There is no error anymore.
But the user data and roles which should be included in the snapshots don't show up after import. Do I have to look out for something special?
Users and roles must be imported separately. The .archive file (or .BIMProject) contains only the project related database files. So if you have e.g. a BIM Server 21, then open its browser based Manager interface, navigate to the Users/Roles page and export these, then import them into your BIMcloud Basic. Also if you used custom access entries for a project, you must set these manually.