Blackscreen when using login for different provider


I am using the Telekom Magentacloud in Germany and after the installation of Nextcloud I cannot log in into my TelekomCloud because the Nextcloud program shows me a black screen. The black screen appears after the part where I can choose to login with an Nextcloud account or a different provider.
I tried attaching an image of the blackscreen I see in the program, but somehow I always get an error uploading using the uploadfunction in this forum.

Thank you for your help