|We've seen this issue first hand (while developing SE 6.0beta), and discussed it at that time in detail with Microsoft Engineers on an IIS 7 Forum.
In the end, New Atlanta decided to let our users be the ones to configure their IIS 7 as desired.
In other words, we decided not to have the SE 6.0 installer try to do anything fancy with IIS 7 to prevent this issue.
Otherwise we'd no doubt have some folks complaining that our installer had broken/changed their IIS 7 error handling behavior, and they would be correct.
We don't see that the IIS 7 Management console offers the ability to set the "passThrough" setting.
So you may simply need to use the AppCmd.exe as the forum post referenced above suggests.
When we used that command on our virtualized Win 2008 we found that it updated our
C:\Windows\System32\inetsrv\config\applicationHost.config file, changing:
(Note: the closing </httpErrors> tag is not shown in the snippet above)
Here is one example of how the passThrough "flag" might be used in applicationHost.config:
If you don't feel like using AppCmd you could always manually edit the applicationHost.config file directly, or perhaps try using a Web.config file with a similar setting.
<location path="Default Web Site">
<httpErrors existingResponse="PassThrough" />
If you do manually edit that file, be sure to use a text editor that is running in an "elevated" manner.
For example, don't simply use Notepad, but rather right-click on the Notepad icon and choose "Run As Administrator",
so that it runs with Adminstrator priviledges.
You must do this even if you are logged into Windows as the Administrator.
Otherwise you'll make edits to that file and they will seem to have been saved, when in truth they were not (closing the file and reopening can reveal the truth in that scenario).
That should help you to get your IIS 7 configured to get out of SE's way in regards to Error Pages.