Mike1605 could you provide kodi logs with "debug switched on" as your issue seems similar to my recent experience (I explain) been using ExpressVPN successfully with "VPN Manager for OpenVPN" for along time now on rpis. However more recently never been able to setup cleanly on freshly prepared rpi with latest version of LibreELEC installed, getting what I recollect is same as you "Auth error with a user, pass and cert/key"
As I had other rpi devices with ExpressVPN setup and working with VPN Manager for OpenVPN installed I got around issue by copying folders and files from working device.
I also firmly believe I was using correct credentials "checked and re-checked" and even compared key files and addons setup files between working/non-working devices checking that username and passwords in files were same/correct etc. (Used manual method and wizard, checked information with expressvpn help team, I could not get to bottom of what I was doing wrong with clean install)
I will re-test again in near future on a new rpi4 I recently purchased and if addon doesnt work I will post kodi.logs with debug switched on in the VPN Manager for OpenVPN addon and Kodi's settings.
