I think from corrections I've read the 'Regular's' post; that interface name needs to be lowercase, and the references to it should be lower case as well.
As it is now
config member 'wanb_members'
option metric '2'
option weight '2'
option interface 'WANb'
config interface 'WANb'
option enabled '1'
option initial_state 'online'
option family 'ipv4'
...
Corrected
config member 'wanb_members'
option metric '2'
option weight '2'
option interface 'wanb'
config interface 'wanb'
option enabled '1'
option initial_state 'online'
option family 'ipv4'