The MinGW-W64 GCC compiler worker executables (cc1*.exe, f951.exe etc.) dynamically link the WinPthreads library winpthread-1.dll, however this DLL is not present alongside those executables as it should normally be.
In some specific cases, for example when winpthread-1.dll is not loaded prior running the worker and it can’t be located through PATH, the GCC driver aborts complaining about failing to start the worker even if the DLL is present alongside the GCC drivers (gcc.exe etc.).
Putting that DLL alongside the worker executables solves the issue.
The problem should be reproducible on a wide range of builds since the MinGW-W64 links the WinPthreads library dynamically by default.
Sigh... This is not a bug with mingw-w64, please contact your binary provider to fix it.