Thank you very much Peter for all the information!
But it is not reported as a backlink to my site. It is reported as one of the top pages on my site.
There is a tab "Top External Inbound Links" there are I can see links pointing to our site. There I would expect that weird link, but it is not here.
Next to it is second tab " To Pages by authority" there I can see my home page, and other pages from my site, divided by authority, but the second is that weird link.So it is clearly stated by moz craw as the second top page on my site and not as backlink. As matter of fact, it also shows that the weird link has 163 backlinks pointing to it!
That is what does not let me sleep over 3 days:-(
Best wishes
Iris