Two SAZ files (one from the client Fiddler instance and the simultaneous one from the server Fiddler instance) would probably help us see exactly what is going on here.
Having said that, you could probably fix this by going to your server instance of Fiddler and in the QuickExec box type each of the following lines and hitting enter after each:
prefs set fiddler.network.timeouts.clientpipe.receive.initial 115000
prefs set fiddler.network.timeouts.clientpipe.idle 115000
Restart that instance.
On the client instance of Fiddler, in the QuickExec box, type the following line and hit enter:
prefs set fiddler.network.timeouts.serverpipe.reuse 100000
Restart that instance:
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