Hi felix and welcome to the Mattermost forums!
The import instructions have been modified recently to reflect the necessity for the subfolder, please see this link.
It’s necessary that the “data” folder is directly in the root of the ZIP file, not sure if your screenshot confirms that fact, looks like an OSX screenshot and I’m not sure if I interpret it correctly.
Can you run zipinfo mattermost-bulk-import-test.minimal.zip to verify that?
The error message “We could not count the users” is misleading and will be fixed in one of the next versions. The default error message is not handled properly unfortunately.