[Solved]
Just encountered the issue with UPS shipping plug-in today.
Got error messages in logs:
UPS Service is currently unavailable, try again later.
Updating the URL in Configuriation->Shipping->Shipping Providers->UPS solved this problem.
Old URL: https://www.ups.com/ups.app/xml/Rate
New URL: https://onlinetools.ups.com/ups.app/xml/Rate
Guess they have been using redirect up to now, but the redirect is getting unreliable.