And when running it uses 100% CPU
(writting this before my PC crashes)
You overloaded your scan task, try with default values and Full & Fast.
You overloaded your system, with 20 Task and 4 concurrent NVTs = 80 parallel running tests.
If you run as much parallel you need a better IO performance or you will block your system and can´t work any more interactive with your PC.
As well heavy kernel scheduling and tuning is needed to run a system with that load. I suggest you reduce the max concurrent hosts to 4 and see how your installation perform that way.
Note that such a 100% CPU usage is the expected behavior during scans, especially Redis is quite CPU resource intensive.
But if a system is crashing under high system load this usually could also point towards issues in the underlying hardware: