Connection does not get established [federation to servers with expired certificate]

I was trying to connect a profile in the default chatmail relay (nine.testrun.org) with a profile in the chatmail relay danneskjold.de, but they do not connect. I get

Even no space to write, nothing. Indefinite. The WiFi is fine for both. Changing the second profile to the default server works. However, adding a second relay chat.in-the.eu does not have any effect

https://danneskjold.de/ has an expired certificate, likely needs an update.