Does abc123_svc_OSAT have access to directory that hosts your ASPX files? The log files you are looking at are only for logging access to your website. You will want to check the event viewer to see the actual IIS error.
Looks like that user does have access to the ASP files. I found this error in the event viewer (thank you for pointing me there): "Application pool ASP has been disabled. Windows Process Activation Service (WAS) encountered a failure when it started a worker process to serve the application pool."