No server connection of the GMX mailbox on the laptop?

I downloaded the GMX Mail app from the Windows Store on my laptop (Windows 8).
After installation, I was able to log in normally, but then a field on the screen with the following error message "popped up":

"currently no connection to the mail server can be established"

and the entire app is empty, so to speak. Can someone help me with the problem?
Maybe someone has had the same problem before.

Thanks in advance!

There's probably a technical problem with your WLAN or with GMX. Try again later.

Check the settings of the (Windows) firewall & permissions of the "app"