BrokenHalo

Over the last week the lock-ups occured more rapidly, so in addition to the things mentioned above I did a lot of testing, even swapping the CPU.
After a lot of digging I found that either of two things seem to coincide with the lock-ups.
So for anyone reading this, maybe this helps you too:
 
SSH-Connections using the SSH-Plugin
For a while I have been using this SSH-Plugin to have a more granular control. In the logs I found messages regarding SSH-connections occuring shortly before a crash/lock-up.
So I deactivated and eventually removed the plugin and moved to this docker-solution, which has been working flawlessly and is even a bit better for what I need.
  PCIe-Confusions
I'm using a GTX 1050 Ti and an LSI Host Bus Adapter as PCI-Devices with the lanes split as 2x8. After the lock-ups the display sometimes showed some cryptic error messages containing PCI-specific values. So I swapped the cards around some and allocated a full 16 PCIe-Lanes to the Graphics Card with the HBA using the remaining 4 Lanes. This seems to have done the trick.
     I have since gotten a few days without any hickups so I hope this has solved the problem for good.