You can check logs for the time where the problem occured, but did you also check v6 connectivity on other devices?
Since the problem is already (for now) fixed, it's hard to find out anything. Only the logs remain, no idea what to look for, as the problem can be manifold...Did you check if clients still had (public) v6 addresses? If it occurs again I would first check this, then looking at the sense´s WAN and LAN interface: is there a public v6 assigned? If so, start a traceroute from sense (or client if it still has a v6 address) to somewhere in WAN (eg google.com). Also try to ping or traceroute v6 IPs on WAN (and sense itself), to see if it is just a DNS problem.