A few things to run through as well, next to what Daniele has already suggested:
Compare behaviour with differentbrowsers and from different machines
Add the QlikView AccessPoint URL to the IE TrustedSites
Are cookies enabled? Cookies are used in the sign in process
IE has problems with host names that include underscores (_), which breaks cookies. Though then I'd expect a Loading Content loop.
Any errors in the browser debugtools / console?
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