The fact that visiting http://ipv4.fiddler:8888/
shows an error message but the direct IP does not means that Fiddler is listening but your proxy settings are not pointed at Fiddler.
I assume you see "Capturing" at the bottom-left of the Fiddler window?
Often, this means that your system administrator has set a group policy which blocks the changing of the system's proxy setting. On Fiddler's LOG
tab, what, if anything, do you see?
When Fiddler is running, what do you see inside IE's Tools > Internet Options > Connections > LAN Settings?
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