I don’t have the error message right now (I applied a quick fix), but I initially received a PHP error that xss_clean() function did not exist at line 212 in ext.safecracker.php.
I opened the file and noticed the xss_clean was not prepended by the global ee instance. I simply changed all instances of xss_clean() to $this->EE->security->xss_clean().
PHP errors when away.
However, my license key will not save now.