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Dominik, whenever you change URLs (domain names) from one to another, there are going to be issues that come up like this. When you redirect one domain name to another you pass on all of the history, links, and soforth to the new domain name. When you redirected the old domain name to the new one you combined the links of both domain names. And the new domain name could have a link profile that is not very good (or that was worse than the old domain name). I would take a look at all of the links of both domain names and perform a link audit to make sure that there aren't any links that you need to get removed or cleaned up.