// During cooking, some platform cant retrieve the MAC adress, so force a default one.
check(IsRunningCommandlet());
* Check to see if this executable is running a commandlet (custom command-line processing code in an editor-like environment)
#if WITH_ENGINE
return PRIVATE_GIsRunningCommandlet;
#else
return false;
#endif
A reddit thread covers this, something about left over network adapters causing problems with the MacAddress. (They mention virtual ones but might also be old hardware specific)
5.1.1 also crashed with the Error "Assertion failed: MacAddress.Num() == 6. Found a thread that talked about fixing that by removing leftover virtual adapters. Had a dozen of those, removed them and got 5.1 to run. Got curious, started up 5.2 and TADA - it works!