Meanwhile I’ve testet the installation on Windows 7 where I have the same issue. After that I tried the oc-client which can be installed without problems.
Windows 7 and Windows 10 are virtual machines under parallesl desktop for mac. Although I never had problems with installing software, perhaps nextcloud doesn’t like this virtual machines for some reason?
I could continue using the oc-client for my nextcloud. It works. But I would prefer to use the nextcloud client.