Thanks Sebastian for sharing your finding.
I finally found the mistake: My VMWare ESX server for virtualization is not connected to the internet, my local PC was. It didn't come to my mind ever before.
I too had the same problems. I use a VMware workstation for my platform. Sometimes I work in a "Local host" only mode and than I have no Internet connection and the mentioned problems.
I verified it now. Fine and thanks again.
Although - I never saw it documented anywhere, that OpenXCAP needs internet access for downloading the namespace files.
I would _really_ recommend to add this to the installations instructions on openxcap.org.
I fully agree.
-franz