"Shared library "Cloud Music Library" is not responding..."

[Otto Nikolaus]

Yes, a lot of people are reporting this. I tried yesterday, and mine was working fine, but apparently a number of Apple servers have been down in the past couple of days.

I've got this today, for the first time ever. I'm ignoring it, on the assumption that Apple will fix it very soon. The reference to a firewall blocking port 443 is simply bogus and misleading, whatever the actual cause of the problem.