Opencv hangs in cv::cvtColor

no, there is probably nothing wrong with your install.
those plugin dlls are optional, and not supplied here.
the info messages are also only shown in debug mode.

cout << getBuildInformation();

will show, which parallel support was builtin.

I have no folder named c:\build\master_winpack-build-win64-vc15\opencv !

this is from the machine, the libs were built on, not yours.