When I start the modpack it only opens vanilla minecraft. But when I exchange the modpack.jar file with one from another 1.12.2 modpack it works. That could be a work around but I want to have a new forge version and know which version it is exactly. Because I couldn't start the forge-universal file to extract it I just took it as modpack.jar as someone in the official guides comments suggested. That could be the problem.
thanks in advance, jolo