Umbraco logs a great deal of helpful debugging information to its /App_Data/Logs subdirectory. If the log files aren’t present or haven’t been added to since a server migration, check that the App Pool user has modify access to the directory tree.
If your Umbraco application is running in an Application pool named “Umbraco Blog Dev” the automatically-generated App Pool user would also be named “Umbraco Blog Dev”
If the”Umbraco Blog Dev” user is missing entirely, add it as “IIS AppPool\Umbraco Blog Dev” in the usual permissions dialogue and add modify access afterwards.
Note: the “IIS AppPool…” prefix is essential, as is the server-name Location scope; Windows won’t find the virtual user without both present.