I just had a similar issue with a COMPLETELY different resolution. Usually when you get errors regarding 'No more AP Manager IP addresses remain' or 'Did not get join response' it means things like:
1. Not allowing SSC
2. Improperly configured AP Policies
3. Wrong time on controller
4. Active interface on ap-manager set to 0 instead of 1 or 2
5. and many others...
Here was my situation: I am setting up a small network with a 4402-25. I wanted to collapse all of my 'wireless' devices onto the same subnet. Without thinking it through, the APs and the wireless clients being on the same subnet resulted in the following layer 3 error:
'Discovery request received on wrong VLAN '253' on interface '1', management VLAN = '0' - in L3 mode, dropping the packet.'
So in my case, you can't have your infrastructure APs on the same subnet as clients on an interface configured on the controller that is not a dynamic ap management interface. IOW, this would not apply to having APs and the ap-manager on the same subnet. It isn't recommended to have the APs and ap-manager on the same subnet, but it is valid.