Between 04/24, 11:00 UTC till 04/24/2018, 16:30 UTC, a small number of customers may have experienced failures in Availability Web Test results executing from these regions : East US, North Central US, Brazil South, South Central US & West US. We root caused this to a backend resource utilization problem which has now been mitigated and all systems are back to normal from 04/24/2018 16:30 UTC.
We understand that customers rely on Application Insights as a critical service and apologize for any inconvenience. We recommend customers to ensure their availability tests are running from at least three locations to ensure redundant coverage. Please refer to the following article on how to configure availability web tests: https://azure.microsoft.com/en-us/documentation/articles/app-insights-monitor-web-app-availability/
-Deepesh