When I export the cloth as apx file an error message says "non orthogonal bind pose".
It usually means that one of your bone has a scale on it. Please check if it's the case.
Here is the mel script to make sure no scale is applied on any bone:
string $joints[] = `ls -type "joint" -long`;
print $joints[0]; //the first joint
* loop through all objects */
string $joint;
for ( $joint in $joints ) {
setAttr ($joint + ".scaleX") 1;
setAttr ($joint + ".scaleY") 1;
setAttr ($joint + ".scaleZ") 1;
};