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The website linking to you shows a low spam score. I don't think this is the cause of your problems. Seems you are looking for a cause of the ranking drop but I doubt it is because of this website with a low spam score. I can't read Polish but just analyzing the site that is what it is telling me.

Thank you James. We are just keeping things as they are right now and working on backlinks. Any more ideas from others on this topic? For what it's worth I also discover a potential bad/penalty link. I asked another question about this here.