Installed WSUS on our App server. Now sage 200 fails to run

I got asked to test wsus on our app server. it installed fine. Hours later the finance department complained of failure to log into Sage (there was strangely a gap).

Now Sage is essentially not seeing the page it wants or the config has been changed. Would WSUS do something like that?

I'm now seeing:

HTTP Error 500.19 - Internal Server Error

The requested page cannot be accessed because the related configuration data for the page is invalid.

Most likely causes:
•The worker process is unable to read the applicationhost.config or web.config file.
•There is malformed XML in the applicationhost.config or web.config file.
•The server cannot access the applicationhost.config or web.config file because of incorrect NTFS permissions.

Things you can try:
•Look in the event logs for information about why the configuration files are not readable.
•Make sure the user identity specified for the application pool, or the authenticated user, has the required permissions to access the web.config file.

Detailed Error Information:

Module
   DynamicCompressionModule 
Notification
   SendResponse 
Handler
   StaticFile 
Error Code
   0x8007007e 

Requested URL
   http://localhost:10082/web.config 
Physical Path
   C:\inetpub\Sage 200 Self Service\web.config 
Logon Method
   Not yet determined 
Logon User
   Not yet determined 

Any ideas? What does WSUS do with IIS that might kill other apps?