It looks like a bug. I have tried from Google Chrome, Firefox and Microsoft Edge on Windows and MacOS. The redirection was the same in each case but the reason was different. I used the Developer Tools to see HTTP requests. It tries to call an API using a wrong docker id (it might be right if a hash of the the real id is allowed there) and gets “account not found” as response or sends the request without any id and gets “offer is not available” as response.
I submitted an issue here: Redirection to Docker Hub home page when linking or re-linking accounts: "Offer is not available" or "account not found" · Issue #2183 · docker/hub-feedback · GitHub
Update:
I have got an answer from mikeparker:
This is a known issue we’re solving this now. See Docker Systems Status Page for details
Update 2:
I realized it was already reported in an other topic:
I should have checked that before I spent so much time to debug it 
I close this topic. Let’s continue the discussion in the other topic if it is necessary.