No rights to access any files

first the details.
I am using a Windows based UrBackupServer 2.5.26. The client is version 2.5.21 on Windows 10. The backups are running fine. If I want to restore a backup under the client, I click on Access/restore backups. The unusual thing now is that I get the error messages “No rights to access any files.” first, then additionally the message Error getting server URL. Cannot access files.
No rights to access any files
Error getting server url

The “Server URL for client file/backup access/browsing” is already set on the server side.
You may have an idea where the error is. This error occurs on all clients except one, an older client. On the one exception where the call works, the browser then opens and there is also a message “Access denied”. After reloading the website, you are redirected to the login so that you can log in normally. Presumably both have something to do with each other. See the added screenshots.