I think I'm hitting the same issue. connman sees my 5ghz network on a scan, but fails to connect. According to journalctl, this error seems relevant:
"wpa_supplicant[523]: wlan0: CTRL-EVENT-ASSOC-REJECT status_code=16"
I am yet to try enabling n/ac mode on the router, will report back once I do.
Update: I am seeing the same as TDown, with ac/n mixed mode I can connect. With AC only I cannot. In both cases WPA2-PSK with AES is enabled. I tried setting the security settings to "auto" but leaving it as AC only and that did not work. I have also tried setting the 802.11 regulatory domain via a file in modprobe.d