IIS 8 folder url and permissions

Note the two lines you have:

er.exe:2240 OPEN C:\WINDOWS\debug\UserMode\ChkAcc.log ACCESS DENIED MY-SERVER\IUSR_MY-SERVER  
er.exe:2240 CREATE C:\WINDOWS\debug\UserMode\ChkAcc.log ACCESS DENIED MY-SERVER\IUSR_MY-SERVER 

The first line might be the application checking whether it exists, while the second line makes for a "create file if it does not exist" situation. The things I would look for is that the IUSR account is not part of the "USERS" group, but is part of "GUESTS", and generally guests have even less privileges than "USERS". You may want to ensure that the IUSR account have the ability to TRAVERSE the folders above it. Try giving the IUSR account explicit access to traverse the folders, and the permission on the "UserMode" folder to "Create Files" (but "THIS FOLDER ONLY").

Finally, there maybe explicit "deny" permissions for guests that may prevent you from accessing the folders. Those are the things I would check.

This sounds like a user access control problem.

With UAC turned on you are not a member of the Administrators group even if you log on as local administrator. You only "belong" to that group when you "run as administrator". So if you launch notepad "as administrator" you will be able to edit the files with no problem. So the answer is to either turn off UAC or grant your user account explicit Modify permissions to the file. Yes it is really annoying.

So that fixes the problem of you editing the config files, but it sounds like you want the app to also edit files. That sounds a bit dangerous, but probably acceptable if you are controlling who has access to the application.

The best approach is to put the web app in its own application pool not the default app pool. Then set the identity on the app pool to a specific user account, not the default network service account. Then grant that account the permissions it needs on the specific files and/or folders.