Dynatrace stores browser monitor screenshots in Amazon S3. If you can't see your reference or error screenshots, either your ActiveGate or the browser from which you access Dynatrace cannot access the S3 service. Change your firewall configuration to allow connections to:
ruxit-synth-screencap.s3-accelerate.amazonaws.com for screenshot storage.
ruxit-synth-screencap.s3.amazonaws.com for screenshot access and viewing.
You can also set up your proxy to connect to the Amazon S3 service. Add proxy connection details to the [synthetic] section of the ActiveGate custom.properties file. For more information, see Set up a proxy for private synthetic monitoring.
For more in-depth information on troubleshooting, see also this Community article on missing screenshots.