Rapid red blinking & reboot

Slightly odd issue follows.
I came back to a Particle project after quite some time away form it being active and encountered a problem where I couldn’t add WiFi credentials to connect to the WiFi AP I had been using for all previous iterations.
On our project we had implemented the custom softAP as detailed in the documentation. Devices that already had credentials seemed OK to connect to to the AP but I noticed periodically there would be a brief orange blip and then the connection would be renegotiated. To investigate this I cleared the credentials and tried to set them up again. On the original Photon I was looking and a fresh one out of the box unprogrammed as soon as I selected an AP I wanted to setup I got an extended series of red blinks (too many to count exactly what it was telling me, so I’m going to guess it was the 13 blink code with anther one following or preceding it) followed by the unit rebooting.
Attempting using the app/SoftAP or setup using CLI had the exact same result, choose the SSID, unit reboots. Interestingly the app sometimes tries to tell me the AP is an enterprise access point (it isn’t its an ASUS RT-N56U in AP mode with 20Mhz selected and WPA2-Personal). To my recollection I haven’t changed anything on it since I was last using Photons connected to it. However since then I have updated firmware, rebooted it and the router handling DHCP and it apparently remains a complete no-go. Instead I have had to resort to using a different much older AP.

Has anyone else seen any similar behaviour, clearly something on my network has changed but what could upset the Photon so much it reboots before it even has credentials?