iMac does not go into sleep mode. Only black background with active mouse pointer

Hello everyone, I have migrated a 9-year-old iMac to a current iMac with current MacOS. The transfer etc. went smoothly. Of course, certain programs had to be deleted, but everything is working. However, there is one problem. When I put the iMac into sleep mode, the background turns black and I only have a mouse pointer. I also can't wake the iMac up from this mode. Can anyone give me any tips?