You're trying to run a browser test in Splunk Synthetic Monitoring and finding that the check is failing. You have already established that the failure is not due to an issue your users are experiencing in your environment but rather something is going wrong within the check itself. You need to be able to diagnose exactly what is happening and fix the issues you're experiencing.
This video shows the steps you'll need to take.
You might be interested in these additional processes related to running Splunk Synthetic Monitoring browser tests:
Splunk OnDemand Services: Use these credit-based services for direct access to Splunk technical consultants with a variety of technical services from a pre-defined catalog. Most customers have OnDemand Services per their license support plan. Engage the ODS team at OnDemand-Inquires@splunk.