There is a bug in /mnet/xmlrpc/server.php line 504
$includefile = $CFG->dirroot.'/'.$filename;
$includefile = '/'.$filename;
First, $includefile is used in mnet_server_invoke_method line 506.
Then $includefile is called at mnet_permit_rpc_call at line 691.
The function, mnet_permit_rpc_call, is in /mnet/lib.php.
In /mnet/lib.php line 414,
The system checks whether there is the file you want to include or not.
But, $CFG->dirroot is atatched again, here.
So, if you want to call /filename, the system checks that whether $CFG->dirroot.$CFG->dirroot.'/filename' exists or not.(doubled!)
Moodle isn't work when you access some function that isn't suported as normal function at setting mnet_dispather_mode='dangerous'.
And it returns RPC_NOSUCHFILE error.