Hi @Maarten Clauwaert ,
Apologies for the delayed response. It was an ah-ha moment when it was pointed out to me. Application Insights uses EventSource for dotnet. Microsoft.AspNetCore.Hosting|Failed Requests
is being generated by RequestFailed() in HostingEventSource. This method is called by HostingApplicationDiagnostics.RequestEnd() when the request ends in a 500. Modifying dataset in your kusto query to the following should pull those 500s in.
let dataset=AppRequest //requests is a legacy table https://learn.microsoft.com/en-us/azure/azure-monitor/app/apm-tables
| where timestamp > start and timestamp < end
| where resultCode > "500"