Same happening here David. I can't replicate it either, it's completely random. Only things to go on are the different terrains players placed on and that the server saved just moments afterwards. I'll keep trying to find some commonality between reports but so far I've got nothing just yet.
Mine's happening with minicopters:
Exception while calling NextTick callback (ArgumentNullException: Value cannot be null.
Parameter name: key)
at System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) [0x00008] in <f98723dd4586469db5213ec59da723ca>:0
at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x00000] in <f98723dd4586469db5213ec59da723ca>:0
at Oxide.Plugins.PlaceAnything+<>c__DisplayClass7_0.<OnEntityBuilt>b__0 () [0x0000c] in <621d60da015246adb4ee426829af03ba>:0
at Oxide.Core.OxideMod.OnFrame (System.Single delta) [0x00051] in <beb2b64691c64e2b95b99491bd85442c>:0