If you happen to get this error page when trying to visit any of your BBIS pages check your app pool settings.
I had just installed BBIS 2.9.1001.0 and I believe that my app pool from later version was still active. So unfortunately the installer just used it. At first I just thought it was the targeted .Net Framework, but after further researching it seems that this error comes from the “Managed pipeline mode” option on an app pool. I switched it from “Integrated” to “Classic” and voila! After I switched it and reset IIS everything seemed to be in order.