Found the problem. The mod 'Better Traders' is a hard incompatibility. Will test more when I have the time.

I stand corrected. After removing better traders they had inventory for a bit, then it happened again without any mod changes

Exception while generating thing set: System.ArgumentNullException: Value cannot be null.
Parameter name: type
at System.Activator.CreateInstance (System.Type type, System.Boolean nonPublic) [0x00003] in <eae584ce26bc40229c1b1aa476bfa589>:0
at System.Activator.CreateInstance (System.Type type) [0x00000] in <eae584ce26bc40229c1b1aa476bfa589>:0
at (wrapper dynamic-method) Verse.ThingMaker.Verse.ThingMaker.MakeThing_Patch1(Verse.ThingDef,Verse.ThingDef)
at RimWorld.StockGeneratorUtility.TryMakeForStockSingle (Verse.ThingDef thingDef, System.Int32 stackCount, RimWorld.Faction faction) [0x0009d] in <b64badbf3c3d41018b3ca5d3e8c77771>:0
at RimWorld.StockGeneratorUtility+<TryMakeForStock>d__0.MoveNext () [0x00085] in <b64badbf3c3d41018b3ca5d3e8c77771>:0
at RimWorld.StockGenerator_Category+<GenerateThings>d__4.MoveNext () [0x00117] in <b64badbf3c3d41018b3ca5d3e8c77771>:0
at RimWorld.ThingSetMaker_TraderStock.Generate (RimWorld.ThingSetMakerParams parms, System.Collections.Generic.List`1[T] outThings) [0x000e4] in <b64badbf3c3d41018b3ca5d3e8c77771>:0
at (wrapper dynamic-method) RimWorld.ThingSetMaker.RimWorld.ThingSetMaker.Generate_Patch1(RimWorld.ThingSetMaker,RimWorld.ThingSetMakerParams)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
(wrapper dynamic-method) RimWorld.ThingSetMaker:RimWorld.ThingSetMaker.Generate_Patch1 (RimWorld.ThingSetMaker,RimWorld.ThingSetMakerParams)
RimWorld.Planet.Settlement_TraderTracker:RegenerateStock ()
RimWorld.Planet.Settlement_TraderTracker:get_StockListForReading ()
RimWorld.Planet.Settlement:get_Goods ()
RimWorld.TradeDeal:AddAllTradeables ()
RimWorld.TradeDeal:Reset ()
RimWorld.TradeDeal:.ctor ()
RimWorld.TradeSession:SetupWith (RimWorld.ITrader,Verse.Pawn,bool)
RimWorld.Dialog_Trade:.ctor (Verse.Pawn,RimWorld.ITrader,bool)
RimWorld.Planet.CaravanArrivalAction_Trade:Arrived (RimWorld.Planet.Caravan)
RimWorld.Planet.Caravan_PathFollower:PatherArrived ()
RimWorld.Planet.Caravan_PathFollower:StartPath (int,RimWorld.Planet.CaravanArrivalAction,bool,bool)
RimWorld.Planet.CaravanArrivalActionUtility/<>c__DisplayClass0_0`1<RimWorld.Planet.CaravanArrivalAction_Trade>:<GetFloatMenuOptions>b__0 ()
Verse.FloatMenuOption:Chosen (bool,Verse.FloatMenu)
Verse.FloatMenuOption:DoGUI (UnityEngine.Rect,bool,Verse.FloatMenu)
Verse.FloatMenu:DoWindowContents (UnityEngine.Rect)
Verse.FloatMenuWorld:DoWindowContents (UnityEngine.Rect)
Verse.Window:InnerWindowOnGUI (int)
UnityEngine.GUI:CallWindowDelegate (UnityEngine.GUI/WindowFunction,int,int,UnityEngine.GUISkin,int,single,single,UnityEngine.GUIStyle)