Yeah, title explains
could not get https://maven.siphalor.de/de/siphalor/nbtcrafting-1.19/maven-metadata.xml

Indeed it is 👍

The server had some planned maintenance earlier today and since then hasn't been reachable.
I filed a support ticket, but as I am extremely busy right now I can't really do anything more right now.

I'll try to report back when the issue is resolved.

Looks like it's back up again