After installing draw.io for Jira Server (Atlassian Marketplace), selecting “add draw.io” does nothing and there are 404 errors in the browser console.
The most common reason for this problem is the base URL in JIRA is incorrectly set. Setting the base URL correctly in JIRA in compulsory, many plugins will fail to operate without this being done.
Fore more general info on draw.io for Confluence or Jira, please refer to our draw.io webpage.
And some great blogposts! Blog - draw.io