Vigor 2866 OpenVPN stopped working after 4.5.3 firmware update

I've been using OpenVPN to connect my Android phone to my music server at home for well over a year.  Recently this stopped working.  I only use the VPN connection once every few weeks but it looks like it may have broken after I updated my DrayTek 2866Vac router to firmware v4.5.3 in June (or possibly to v4.5.2.3 which came out only a couple of days before 4.5.3).  I haven't (knowingly) changed anything to do with VPN on my phone or router recently.

When trying to connect my phone's OpenVPN app displays this message before giving up:
AUTH: Received control message: AUTH_FAILED,DHCP assign IP error.

The 2866Vac system log shows this:
 2026-07-02 07:30:05      OpenVPN (VPN-0, 31.94.36.178) HARD RESET V2, start negotiation
 2026-07-02 07:30:06      OpenVPN (VPN-0, 31.94.36.178) Remote option is not matched
 2026-07-02 07:30:07      OpenVPN (VPN-0, 31.94.36.178) AUTH_FAILED,DHCP assign IP error.

The phone connects using the "OpenVPN for Android" app (last updated 2026-01-30) via EE's 4G cellular network.

Does anyone have any ideas what has gone wrong and how I might fix it?  I do remember it took a lot of faffing around before I managed to get OpenVPN to work, and I can't remember the exact steps I took, so I'm not keen to scrap the setup and create a new one from scratch (especially if OpenVPN connectivity might be completely broken on the 2866).