I've found depending on how your ports are setup (Trunk / Access) and vlans assigned, you may need to leave the vlan field empty when assigning a static IP.
So, if you Vlan1 is the default and you're on a trunk port - I need to leave the VLAN number off of the WAPs, and leave the field blank actually.
On the Switches I do have the VLAN1 (or whatever is correct) in the IP assignment.
This has frustrated me before, since the error isn't specific to say 'bad VLAN' or something more descriptive...