[NextCloud 13] login only in the own network possible

Hello,
I have installed NextCloud from the scratch at a linux system.
All works fine, if Iā€™m in my network at home.
But if I call NextCloud from remote (port redirection via FritzBox!) I see the login screen, but no login is possibleā€¦ Why?

Any hints?

what is happening instead? What do the logfiles on your server tell?