I've setup the whole SharePoint 2013 App Domain stuff many times on different configurations without issues, but now I have one customer installation with some weird behaviour.

If I start an App installed in the hostweb, I get the credentials Dialog for Authentication. I'd expect automatic Authentication. I tried quite a bit, but to no success. Yesterday I found an interesting solution to it. As long as Fiddler is running on my Client, Authentication works as expected.

What does Fiddler do? Any ideas what causes that behaviour?