If you use phpthumbof on a subfolder on your webserver than phpthumbof uses internally the wrong path.
For example:
Your document root ist yourdomain.de and points to /var/www/htdocs/yourdomain.de
Your MODX-Installation is in the subfolder /var/www/htdocs/yourdomain.de/test/
You edit your MODX-test-"domain" with yourdomain.de/test/manager.
-> everything is working as expected (phpthumbof not)
if you create a new subdomain aka test.yourdomain.de as a real document root und map this domain to your subfolder /var/www/htdocs/yourdomain.de/test/ phpthumbof is working as expected!
Keep in mind to clear the MODX-Cache before changing and to modify the file core/config/config.inc.php (only the entries with relative paths!)
Oliver