Thanks for reaching out Dominik,
Thanks for your questions and thank your using Total Upkeep for WordPress!
By default, the plugin keeps the generated backups in a folder located in the root directory of your hosting rather than in the site directory itself. This is to help prevent any issues affecting your sites from having any baring on your backups as well as to prevent your backups from being included in subsequent backups. This setting can be changed but before we go that route we want to get some of the usual suspects out of the way first.
First, find out your backup directory. You can find this in Total Upkeep -> Preflight Check. Most of the time, this is in your home directory, in a directory called boldgrid_backup.
Could you send us the log file of the most recent backup attempt made on your website? I would like to see if our team can find any indicator of what the issue might be based on the information in that file. We have some troubleshooting guidelines in our support center related to backup creation failures and the instructions there will help you locate the Total Upkeep log files.
We look forward to hearing from you!