If your Confluence instance is switched to the new user interface, draw.io will break. This is an Atlassian bug which they tracking here. It's marked as fixed, but that's only fixed internally, it's not yet rolled out. The workaround stated to opt out of the new interface does seem to fix the problem

draw.io for Confluence Cloud is a plugin for Atlassian's Confluence Cloud product.

draw.io provides embedded diagramming functionality to add a range of diagram types to Confluence pages. All diagram data is stored only in Atlassian's cloud, directly from your browser (it doesn't pass through our servers during save/load).

 

End User License Agreement

Service Terms and Service Level Agreement

Security Statement for draw.io hosted on Atlassian Cloud products