Windows Desktop Client timout at connection attempt

Hallo everyone.

I have a problem to connect with the windows desktop clienet to my nextcloud-server from external.
My software versions:

  • server: nextcloud v11.0.1 on Raspi 3 B
  • client: Nextcloud- on Win7 64Bit

Connect from my home-pc, which is in the same network as the nextcloud-server, with the dektop client to nextcloud.
Connect with a browser to nextcloud from external and internal.
CardDAV from my Android Phone is working external and internal.

I used the URL-schema: ( is a dynamic-dns service by AVM)
on every device. External and internal.

Not working:
Trying to connect with the desktop client from external to nextcloud.
This error appeared when I try to setup the connection:

The first words mean: “timout at connection attempt with Nextcloud under http://next…”

Does somebody have a hint where I have to look and how to fix the problem?

Thank you very much,