Summary

When user tries to edit the existing comment on the issue, Jira returns the error: "We couldn’t save your comment. It might be empty or have invalid formatting. If you copy-pasted, try removing the formatting and adding it directly in the editor."

From the network tab of browser debug we can see the following error message:

{
	"errorMessages": [
		"You can not edit the comment as the issue is in a non-editable workflow state."
	],
	"errors": {}
}

Cause

This happens when the Workflow Status Property  jira.issue.editable is set false on the status. This is used disable editing when an issue is on a particular status. Often used on a final status. 

Solution

If you want to edit the existing comment go ahead and set Workflow Status Property  jira.issue.editable to true. The error message on the GUI is misleading, there is feature request to improve the error message 

JRACLOUD-82321 - Getting issue details... STATUS