There is a bug in /mnet/xmlrpc/server.php line 504

$includefile = $CFG->dirroot.'/'.$filename;

I think

$includefile = '/'.$filename;

is true;

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.

Please check!