are your R510 (not R150) listed under "Access Points"? then you set up a WLAN in "Wireless LANs" and put it in wlan group (Default). choose this wlan group for the accesspoint group, make sure all accesspoints joined the group, under "Radio Settings" choose the wlan group for 2.4 and 5GHz. I am sure that you have already done all this right.
i think you have (still) other accesspoints running with the same ssid, then your phone sometimes connect to that "Rouge" accesspoint, and because the passwort does not match, you will be asked for that. and vice-versa. make sure that no other accesspoint uses the ssid from the zonedirector. maybe you still have old wlan routers on the run