I am also suddenly unable to open the webui. Worked up until yesterday. I am seeing AUTH_FAILED and [soft,auth-failure] in my logs. But am quite confident that the current credentials and certs are valid. If I set VPN_ENABLED to false, then I am able to open the webui. Suppose it could be an issue with OpenVPN.
EDIT: randomly started working for me today. Must have been an issue with openvpn sub. Thanks!