iCloud login problem on Sierra, High Sierra? Try this.
My Notes weren't updating on my Macbook Air with High Sierra, so I logged out of iCloud to see if logging back in would fix it, only to find out that I couldn't log back in. Got all of the "Unknown" and "Not at this time, try later errors".
Short story, did all the things, but no success. iCloud is fine, other machines are fine, just not this one. Finally tried logging into iCloud on that machine with Safari and got a "Can't connect to iCloud Servers" error. Ruled out network problems and Firefox connected to iCloud just fine.
Turns out it was a certificate problem preventing this machine from connecting to iCloud. Following the advice in this thread fixed the problem and now iCloud is working correctly again. Download the Apple Intermediate Certificates as mentioned in the first post.
How I solved the Safari 13 and High Sierr… - Apple Community
