Problem with federation sharing

I have another Problem with my Nextcloud. I use Federation to share documents between different Instanzes. This works perfect. But now I have a problem with a share from onely one Server. I can’t Accept or Decline it.

Everytime I click on one of the buttons I recieve the following Message:
“Failed to perform action
Verbindung zum Server verloren”

Can someone help me to fix this?

Thaks a lot!

Nextcloud version (eg, 10.0.2): 12.0.0
Operating system and version (eg, Ubuntu 16.04): Debian
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.10
PHP version (eg, 5.6): PHP 5.6.30

Is this the first time you’ve seen this error?:

Can you reliably replicate it? (If so, please outline steps):

The issue you are facing:
If I click on the “accept” Button I recieve the message
"Failed to perform action
Verbindung zum Server verloren"

The output of your Nextcloud log in Admin > Logging:
Error files_external Sabre\HTTP\ClientException: Failed to connect to port 443: Connection refused

I cant reach the other cloud from my Server with wget. Both Instanzes run with a Let’s Encrypt Cert.