I am running 4 AP160s from a 3448 and having the exact same issue since updating the APs to 18.104.22.168.
Adding more detail in hope for answers or ideas:
I know I am fighting with interference from other companies in the same area, but only on the B side, A is clear (site survey showed only 2 other companies running A and not in the same frequency). Since updating to the newest firmware, the APs consistently lock up and stop responding, this has been daily and often more than once a day, requiring a restart before they will function again. SSID broadcast is also intermittent when this happens, sometimes it broadcasts the SSID but a client will just keep being asked for the passphrase without ever connecting while other times the SSIDs no longer broadcast from the AP.
I have the same issue after upgrading, I will open a case.
I've seen exactly the same issue on 6.9.2 in a single AP, interference-free area. In one instance where the same SSID was on both radios, it dropped into the "problem mode" on 2.4 GHz but 5 Ghz continued to operate properly. Interestingly enough the users complained of a coverage problem. Ha! Sending various config updates had no effect. I had to have someone on-site power-cycle the AP to get it to come back up.
Downgrading to 6.5.5-18 has solved this problem for me, but I have seen in other instances where this rev doesn't play nice in high-interference environments. I have an open case where this came up. No relevant activity yet.