Ubuntu Mate 20.4 (Focal) doesn't not go to sleep mode

My working station (RTX 2080 + Ryzen 3700X) with mate ubuntu 20.04 refuses to sleep. When I do Shutdown/sleep, it logs out of the session but does not actually go into sleep mode. Here are some infos:

foo@foo-workstation:~$ nvidia-smi 
Tue Aug 16 17:59:52 2022       
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 495.29.05    Driver Version: 495.29.05    CUDA Version: 11.5     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|                               |                      |               MIG M. |
|===============================+======================+======================|
|   0  NVIDIA GeForce ...  On   | 00000000:08:00.0  On |                  N/A |
|  0%   58C    P0    58W / 250W |    858MiB /  7959MiB |      4%      Default |
|                               |                      |                  N/A |
+-------------------------------+----------------------+----------------------+

and some more:

foo@foo-workstation:~$ systemd-inhibit --list --mode=block
WHO   UID  USER  PID  COMM            WHAT                                                  WHY                                     MODE 
foo 1000 foo 5605 mate-power-mana handle-power-key:handle-suspend-key:handle-lid-switch Mate power manager handles these events block

What I already did: - Purge all nvidia packages and install nvidia-driver-495. - Read various threads on this topic none of which helps. Any help will be greatly appreciated.