When my end users try to use Fiddler when Proxy Connect Endpoint Client is installed, Fiddler does not record any traffic

This behavior is unavoidable as Fiddler gains visibility by means of changing the proxy settings to a virtual proxy hosted on the local machine. This is interrupted by Proxy Connect Endpoint Client, which will implement its own proxy settings. 

• In order for Fiddler to work, Proxy Connect Endpoint Client must be disabled.

Forcepoint Web Security Cloud customers can set specific users to be allowed to disable this manually by selecting these users to be allowed to do so at Web > Endpoint > End User Control. 
Forcepoint Web Hybrid customers will need to either re-deploy a package with the ability to disable selected during the package build, or have the package manually disabled using the anti-tamper password.

• An alternative to using Fiddler to capture browser traffic is use take a HAR format capture using the browser's network capture tool in their developer tools suite, as this will work when Endpoint Client is running. The HAR format capture file then be loaded into a third-party HAR capture viewer, or into Fiddler itself.  For more information on HAR, see Collecting browser debug logs
• An additional alternative solution is to deploy Direct Connect Endpoint Client on the machines of the users that you specifically require to be able to use Fiddler. This will, however, require the clients to have access to the internet through the firewall of whichever network they are connected to, therefore this may not be suitable for all deployments where connections to the internet must be proxied.

Keywords:
fiddler; har; proxy; pcep; proxy connect endpoint; F1E; interoperability; conflict; browser; client