OK, that did work – however, since (as recommended by https://technet.microsoft.com/en-us/library/cc678863.aspx#SharePoint administrative accounts) the SharePoint farm service account is not a local-admin it cannot assign shell-admin permissions. So at first I had to make the farm-admin a local admin, then start an elevated shell as farm-admin grant the setup-account shell-access and then issue the psconfig-command as the setup-account. And of course retract the farm-admin from the local admins prior to logging out of the system.
Seems like quite som hassel – but, finally it did work – well, after running upgrade-spcontentdatabase on a couple of databases, which didn’t got upgraded properly by psconfig.