So the problem is now solved somehow. I read the upgrade guide, because yesterday i updated flexget to the newest version. Unfortunately i didn't upgrade setuptools as well before that. So, now after following properly the instructions for upgrading, I get back search results from rarbg plugin and everything works fine. So it appears it was simply a step I missed during the upgrade process, though I don't have any idea if this had actually something to do with my problem but all is well now.