Cannot save a Template containing a variable

Repro:

• Create anew Confluence Template and give it some title
• add a variable in it ($myvar)
• save the template

It causes the printError The response from one or more JIRA instances cannot be parsed."

It fails, the template is not saved and the browser returns ERR_EMPTY_RESPONSE.
Looking in the log give several messages. See the following Atlassian Answers thread, it seems many experienced the problem
==> https://answers.atlassian.com/questions/29623410

Repro the same steps without the variable, and it works.