Found the problem. In the apache vhost file of the proxy:
ProxyPass /nextcloud http://192.168.1.20:8080
ProxyPassReverse /nextcloud http://192.168.1.20:8080
instead of
ProxyPass /nextcloud http://192.168.1.20:8080/
ProxyPassReverse /nextcloud http://192.168.1.20:8080/
The last slashes where to much