Curious. Is that on Linux or Windows hosting?
The code tests correct on Linux. Specifically in autonav-wl-options.php there are the two lines:
$realpath = realpath(__DIR__."/readme.txt");
$path = 'https://' . $_SERVER['HTTP_HOST'] . substr($realpath, strlen($_SERVER['DOCUMENT_ROOT']));
which should take the local (from the server’s perspective) path of the readme.txt file in wp-content/plugins/autonav and then remove the prefix part of that local path which coincide with the webserver’s document root. That might break on Windows if your document root were on drive C: but the plugin was on drive D: … or something else equally bizarre.
Perhaps you could find those two lines, and in the WordPress editor, temporarily insert between them the line:
print "ROOT: " . $_SERVER['DOCUMENT_ROOT'] . " -- README: " . $realpath . "<br>\n";
save the file, and report back please with what that prints.