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).

Today I spotted firmware version 4.5.3.1 on the DrayTek web site.  It was released only a few days after 4.5.3 and contains a few security patches and bug fixes.  Oddly, the "Current Version" at the top of the 2866's page on the DrayTek web site still shows 4.5.3 as the latest one, and "Firmware Upgrade" in the router's management web interface also shows 4.5.3 as being the latest.  4.5.3.1 has replaced 4.5.3 in the list of current files available to download further down the 2866's page on the DrayTek web site, though.

After installing firmware 4.5.3.1_BT my OpenVPN problem was not solved.