Verbindungsassitent Fenster ist schwarz keine Eingabe möglich

I use nextcloud for quite a while now but this one is new.

I installed the recent NC win client 2.5.3 on my remote win7 VM.
I am logged in via RDP.

But I cannot connect to my cloud as the Connection-window (nextcloud-Verbindungsassisten) ist black! I cannot put my username and passwort or app-token.
I changed color depth, resolution. logged in via remmina and windows-remote desktop client, but the connection-Windows is simply black…
Does anyone have a clue? (Maybe I register via comandline ?)