PlantUML: "The plugin you are using seems to generated a bad URL." on official PlantUML server

My Setup

• I have a private Synology Nas that ran GitLab version 11.3.0 in Docker and was upgraded today to 12.9.5 AFTER I encountered the issue.
• It used the official plantuml server.

Problem

When I viewed a markdown file this morning, instead of the diagram I saw this:

Steps taken

• First I updated to the latest version. This didn't change anything.
• After that I set up my own plantuml server in Docker. This solved the issue.

I still would rather use the official plantuml server as my own setup is quite slow.

Here is the issue in the plantuml forum:
https://forum.plantuml.net/11432/the-plugin-you-are-using-seems-generate-private-gitlab-server?show=11433#a11433