does your plugin add something like this in ..htaccess file
this is redirecting all .txt files to 404 and causing a bit problem.
although i tried deactivating you plugin, which didn't helped me, still have this doubt.
"RewriteRule ^(((lfdir/wp-content|lfdir/wp-includes)/([A-Za-z1-9-_/]*))|(wp-admin/(?!network/)([A-Za-z1-9-_/]+)))(\.txt|/)$ /nothing_404_404 [QSA,L]"