This issue is related to the fact that Amazon Web Services is going to start charging for IPv4 addresses.
We don't have a single server behind https://www.plantuml.com; there are multiple servers.
Some servers use IPv4, while others use IPv6. It appears that IPv6 servers are experiencing issues with outbound connections to the internet, which are required for !include.
We need to begin our investigation, so please be patient... :-)
Apologies for any inconvenience!