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Hi Anna
I also have this problem. I have tried following your guide but without result.
Tried on two 2018 Macbook Pros running High Sierra, resulting in "Http/1.1 Service Unavailable"
I also have an old MacBook Pro 2008, running El Capitan. Here it works fine. Both Skype for Business and OneDrive.
Hoping for a quick solution
Regards
Casper