@Dominic Calabria - Apologies for your experience on this issue. Thanks again for reaching out to us regarding this issue.
Updating the information from comments here for broader community usage.
Azure Application Insights REST API Explorer is currently returning "Internal Server Fault" , this is a known issue which our team is aware and working towards fixing the issue. It only affects demo_app and will be resolved in next 1-2 buisness days.
8/27 Update:
The issue has been resolved. You can should be able to use demo_app with in Azure Application Insights REST API Explorer.
Please let us know if you any further queries. Thank you