I'm getting the incompatable version error with Valheim+ now.

Valheim plus has not received any significant updates. The mod is broken and as of right now there is no maintenance by the creator.

There are community fixes, but those are just going to be temporary bandaids.

Unless someone commits to maintaining the mod if you want to continue using it you need to research how to play without updating the game.

https://steamcommunity.com/app/892970/discussions/0/3827537216853346525/
https://steamcommunity.com/app/892970/discussions/0/6620894968762024640/
https://steamcommunity.com/app/892970/discussions/0/6620894968760355510/

Have you updated BepinEx - and have you updated all files on the server?

The last updates broke valheim+, but someone has thrown a fix out. He is on the valheim+ discord and you can also go there to get further help.
You may have to use the windowsserverrenamed asset to fix the server side.

https://github.com/Grantapher/ValheimPlus/releases/tag/0.9.9.13