Old post but i just hit this snag as well and it wasnt a hardware issue which was listed..
After some show commands I noticed I missed some config..
MC# show crypto ipsec sa
Oh the AP has an inner ip of 0.0.0.1
but I made my VPN Pool...
show log all 30.. Okay it uses the default-vpn-role and uses L2TP
Go to the default-vpn-role ->More-> L2TP field is EMPTY. Make sure the L2TP drop down has your VPN Pool selected :D
TFTP is used between RAP and MC btw for image updates
Reboot AP, convert again,. Boom - Correct inner IP and conversion successful.