Still not fixed on xcode/server 11.6. I think it's happening because the server itself isn't authenticated. I'm saying this because I tried whether creating a new bot shows the branch selection step but on the first window of Create Bot wizard, where you select the Bot name and choose the server, the server keeps showing offline.
I tried removing/re-adding Xcode Server account from Xcode > Preferences > Accounts. The server appeared online in Xcode preferences as expected but still when I open create bot window, it appears offline with the small loading view keep spinning in front of the "Server" field on create bot window.