Filemap file does not exist

hi all, having the same issue as a few others, was hoping someone came up with an answer since 2010.  im getting the

HTTP Error 500.50 - URL Rewrite Module Error.

System.ArgumentException: Rewrite file provider exception. The file "C:\IDC_HOSTED\App_Data\redirectmappings.txt" does not exist.

etc etc.

as per other suggestions i have checked permissions on iuser(s), network service, all ok.  file DOES actually exist in the location specified.  enabled tracing for 500.50.  the trace log shows no more info than the IIS7 detailed error screen.  if i remove the filemapprovider, the site works.  if it is in place, i always get this error.

any ideas?