Found the problem :)
In the HTTP security options on the moodle-server (site administration -> General -> security -> http security) the "private" network had been blocked
therefor for everyone who wants to test this in their home network
hint: change cURL blocked hosts list to
10.0.0.0/8
172.16.0.0/12
0.0.0.0
169.254.169.254
0000::1