Although I have disabled update checks by setting the following registry keys I found earlier, Fiddler seems still to check during startup and throws an exception:
[HKEY_CURRENT_USER\Software\Microsoft\Fiddler2]
"CheckForUpdates"="False"
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Fiddler2]
"BlockUpdateCheck"="True"
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Awww, Fiddlesticks!
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Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report at http://www.telerik.com/forums/fiddler.
VersionCheckFailed: Server Response Code = 502
Type: System.IO.InvalidDataException
Source: Fiddler
at Fiddler.Updater.GetLatestVersion(Boolean includeBetaVersions, Boolean isEmergencyCall)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
at System.Threading.ThreadPoolWorkQueue.Dispatch()
Fiddler v4.6.2.3 (x64 AMD64) [.NET 4.0.30319.34209 on Microsoft Windows NT 6.3.9600.0]
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OK
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