Hi Christian,
I do not understand this part :
"
Testing already fails in SICF.
But only for the "Reference"-Service itself.
If I test the source (=reference) it works.
"
Maybe a screenshot like the below will make this clear:
You need to make sure that the service is testing fine from /nSICF; only then you will be able to call it from portal.
Regards,
Hemanth