IIS app now requires physical path credentials

Question

I have an application developed using ASP.NET Web API deployed to a site in IIS. Recently the application began crashing during initialization due to an exception saying that it could not access a particular folder (where it writes logs to). The app pool being used by the site uses a domain account and I verified that that account has full permissions on the folder, just as it always has. What wound up fixing the problem for me was to specify the username and password of a user who has access to the folder in the site's Physical Path Credentials field. Previously this had been set to "Application user (pass-through authentication)".

What could have changed in the application or IIS configuration to suddenly require this field to be set?

Answer 1

It turns out that this came down to a permissions issue. Write permissions on the folder for IIS_IUSRS got changed from Allow to Deny. After switching that back to Allow, I can remove the Physical Path Credentials setting.