I've experienced a fair bit of WebGUI crashes and other shenanigans (Aimesh nodes crashing, my computer connects to the main node where I barely have signal) immediately after firmware update on the later Merlin versions on my AC86U.
Simply logging in and rebooting through SSH via a LAN client solves it for me (if you have easy physical access, you can reboot it manually). If you don't have SSH set up, do this for next time (there likely will be a next time).
I'm not sure if it is reachable through OpenVPN/wireguard in this state, so if you're not on the LAN, you have to try and see (please report back the result).