CentOS - Oct 2021 - anyone her using nexus 3 as a repo-mirror for centos

Hallo
anyone her using nexus 3 as a repo-mirror for centos.
My problem:
yum update ends with error 404 and I ve no idea what is going wrong.
The nexus repo is empty so it is clear why 404 appears but I thought  
that the nexus fetches the files immdeatly from outside.

Thanks for hints
Ralf