My initial advice would be to repeat all the steps in a different browser or/and Incognito mode in order to see if you still have an error.
Also, clear the cache and cookies and temporarily disable all browser extensions.
If this doesn't help, please:
* Tell us what browser/OS are you using
* Describe us all the steps you performed so we can try to repeat them
* Attach us .xml file here so we can test it directly (or send it
sup...@draw.io if you don't want to share the file publicly)
Kind regards,
Marija