Having 4 of these all working reliably, one started to throw itelf into a reboot loop for no apparent reason.

Managed to get into it and do a backup. Got another router and restored the backup whereupon the new one immediately started the reboot loop.

Obviously must be a messed up setting somewhere.

Reset both and recreated all the settings (what a pain in the backside) and Lo ! All is well. The original one seems to be back to its normal self and no reboots.

So the wisdom is

a) make sure you have written down all your settings and don't rely on a backup (fortunately I had)

b) if in doubt, do a factory reset first

If it takes a while like that, I've seen it happen when there's an issue with a VPN breaking a routing table or similar, try booting it without the WANs connected to reconfigure it, which has worked for me before.

If it takes a while like that, I've seen it happen when there's an issue with a VPN breaking a routing table or similar, try booting it without the WANs connected to reconfigure it, which has worked for me before.