Hello Erik,
thanks for your answer. I've checked my settings and they are the same as you wrote. My limit of file versions are 6. I click on 'monitor disk space usage for backup storage' and set the value to 450 GB for an disk amount of 500 GB and I activated 'automatically optimize storage'. Now my backup runs fine and I've furhter questions, then I'll contact you.
Thanks and Best Regards,
Joachim