Now the error is fixed but it seems I cannot use a single plugin. I am running a mostly vanilla server with these installed:
AdminDeepCover.cs
AdminRadar.cs
ConvertStatus.cs
Freeze.cs
GrTeleport.cs
InventoryViewer.cs
NoGiveNotices.cs
PermissionsManager.cs
PlayerAdministration.cs
SteamChecks.cs
Vanish.cs
WipeDataCleaner.cs

And none of them work. The Oxide version is v2.0.4582 and I have updated it today. I installed the server from scratch before installing oxide...

Alright, what does your log say when starting up the server?