This is the answer I found on another forum, but it doesn't make sense to me.
Hey man! i had that issue too, it’s because you need the rig to be rotated, let me show you:
[ATTACH=CONFIG]312332[/ATTACH]
Check out the rotation of the armature, for some reason when you do your own armature you need X and Z to be 90 degrees rotated before export which is facing up positive Z and using X as lateral axis as if the armature were lying on it’s back on the ground.
What i did to fix that is position the armature lying down apply the rotation with Ctrl-A and then move back up to the normal position facing positive X and pointing up to positive Z, using Y as lateral axis.
then, without applying the rotation, export the dae, that should do the trick.