Remote access to custom domain - some problems with cloudflared

Hello,
I’ve a custom domain by checkdomain.de There I’ve added the nameserver from cloudflared.
The tunnel is working and in log I see: Finished setting up the cloudflare tunnel
But while connecting to the tunnel I see different connections and a 'Failed to serve tunnel error error timeout: no recent network activity

I have no clue from networking-topics, so I am totally lost with this error.

What does it mean and what can I do?