OPNSense locked after power outage

[ANaiveUser]

Hello there!

Today we experienced an unexpected power outage in our office that lasted about a minute. Since then our OPNSense (DEC2770) has become mostly inaccessible, at least for administration.

The networking configuration, DNS, and related services seem to be working fine, but I can no longer reach the web UI. Access is restricted to our VPN, and while the VPN itself works (I can connect to other VPN-restricted systems), the web UI remains unreachable.

I also tried accessing it via the serial interface, but it says that my credentials are incorrect, even though I used the exact same ones less than an hour before the outage. SSH isn't enabled.

How can I regain access?