Windows Authentication - specific folder permissions in IIS 8.5 - iis

Question

We have intranet website that is deployed on IIS 6 with windows authentication and ASP.Net Impersonation enabled. It works perfectly, but when we moved to IIS 8.5, logging (to a log file) seems to stopped working. When we ran Process Monitor, it shows access denied to the folder where logs are written. And it also shows that, it is impersonating logged in user to write the logs. Where we want the system to use app pool user to log. I tried everything available on internet, changing entries in applicationhost.config to adding location paths and adding web.config to that particular location, nothing seems to work.
Update:
Just executed Process Monitor on old server and below is the comparison. it is exactly same, except new server denies the access. In both the cases, impersonating user (logged in user) tries the access to folder. I think something to with OS. Old server us Windows Server Standard and new one is Windows Server 2012 R2 Standard.
Old Server
Operation:CreateFile
Result:SUCCESS
Path:XXXXX\log.txt
Desired Access:Generic Write, Read Attributes
Disposition:OpenIf
Options:Synchronous IO Non-Alert, Non-Directory File, Open No Recall
Attributes:n/a
ShareMode:Read, Delete
Impersonating:domain\username
OpenResult:Opened
New Server
Operation:CreateFile
Result:ACCESS DENIED
Path:XXXXX\log.txt
Desired Access:Generic Write, Read Attributes
Disposition:OpenIf
Options:Synchronous IO Non-Alert, Non-Directory File, Open No Recall
Attributes:n/a
ShareMode:Read, Delete
Impersonating:domain\username