I found the solution. It turns out that Sharlikran's remark about AGS is correct. I temporarly removed the AGS addon and now it works as expected. Which is a bit of a pity, because I really want to keep using AGS.
So I guess that means that this is the end of my endeavour to process listing data outside ESO
OTOH I learned that some AddOns apparently change the 'standard' ESO API calls in unexpected ways (I never would have figured this out myself
)
thank you for your help
cabwav