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