While extremely useful, inband doesn't monitor above layer 4. For example, if your server stopped responding or was sending tcp RSTs, you'd see it flagged as down. As it stands, your tcp-level chatter is 'healthy' to the inband monitor - after all, a 500 ISE is an application response of an application event; well above tcp level stuff. On other words: inband monitors don't actually seek out specific application data (here an HTTP response).
For application data like that, you'll want to use an HTTP monitor to deal with the HTTP responses you expect and don't expect.
I hope this helps.
-Matt