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? Thanks in advance.