I fixed it: As the new connector didn't work on 2 server with max. permissions (Global Admin, Enterprise Admin, Domain Admin, Intune License,...) I wanted to install the old connector, but that also didn't work, because somehow my credentials were cached, even after uninstalling connector and rebooting the server, same issue next day, nothing happens when I clicked on "Sign in". Then we removed my user profile from the server and I tried again and it did work (with the old connector). So we moved on to the other server, removed profile, uninstalled legacy connector and installed the new one and this time it also worked ( still used account with max permissions Global Admin, Enterprise Admin, Domain Admin, Intune License,... better save than sorry :)).
Now I have both running old connector on one server and the new one on the other.