I encountered the same issue — after I downed the mechanoid, it couldn't die.

Same

Error while killing Mech_Scyther405536 during phase 4: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
[Ref 4B8CFA4D]
[0x00000] in <51fded79cd284d4d911c5949aff4cb21>:0
at System.Collections.Generic.List`1+Enumerator[T].MoveNextRare () [0x00013] in <51fded79cd284d4d911c5949aff4cb21>:0
at System.Collections.Generic.List`1+Enumerator[T].MoveNext () [0x0004a] in <51fded79cd284d4d911c5949aff4cb21>:0
at VanillaTraitsExpanded.TryGiveThoughts_Patch.Postfix (Verse.Pawn victim, System.Nullable`1[T] dinfo, RimWorld.PawnDiedOrDownedThoughtsKind thoughtsKind) [0x00050] in <0826dd859bf44750a172800a2814bfd6>:0
at RimWorld.PawnDiedOrDownedThoughtsUtility.TryGiveThoughts (Verse.Pawn victim, System.Nullable`1[T] dinfo, RimWorld.PawnDiedOrDownedThoughtsKind thoughtsKind) [0x00235] in <fcfab19af5d14a608a65ebd77f29482f>:0
- POSTFIX OskarPotocki.VanillaTraitsExpanded: Void VanillaTraitsExpanded.TryGiveThoughts_Patch:Postfix(Pawn victim, Nullable`1 dinfo, PawnDiedOrDownedThoughtsKind thoughtsKind)
at Verse.Pawn.DropBeforeDying (System.Nullable`1[T] dinfo, Verse.Map& map, System.Boolean& spawned) [0x00055] in <fcfab19af5d14a608a65ebd77f29482f>:0
at Verse.Pawn.Kill (System.Nullable`1[T] dinfo, Verse.Hediff exactCulprit) [0x00268] in <fcfab19af5d14a608a65ebd77f29482f>:0
- PREFIX OskarPotocki.VEF: Void VEF.Storyteller.VanillaExpandedFramework_Pawn_Kill_Patch:Prefix(Pawn __instance, Nullable`1 dinfo, Hediff exactCulprit)
- PREFIX OskarPotocki.VEF: Boolean VEF.Apparels.VanillaExpandedFramework_Pawn_Kill_Patch:Prefix(Pawn __instance)
- PREFIX vanillafactionsexpanded.settlers: Void VFE_Settlers.Utilities.Harmony+PawnKill_Patch:PreFix(Pawn __instance)
- PREFIX VanillaApparelExpanded.Accessories: Boolean VAE_Accessories.HarmonyPatches:BeltsOnDeath(Nullable`1 dinfo, Hediff exactCulprit, Pawn __instance)
- PREFIX OskarPotocki.VanillaSocialInteractionsExpanded: Void VanillaSocialInteractionsExpanded.Pawn_Kill_Patch:Prefix(Pawn __instance, Nullable`1 dinfo, Hediff exactCulprit)
- POSTFIX OskarPotocki.VEF: Void VEF.Pawns.VanillaExpandedFramework_Pawn_Kill:Postfix(Pawn __instance)
- POSTFIX com.alphagenes: Void AlphaGenes.AlphaGenes_Pawn_Kill_Patch:RemovePocketPlanes(Pawn __instance)
- POSTFIX VFEEmpire.Mod: Void VFEEmpire.HarmonyPatches.ColonistTitleCache+OnPawn:Postfix(Pawn __instance)
- POSTFIX vanillaexpanded.factions.deserters: Void VFED.HarmonyPatches.MiscPatches:ExplodePackOnDeath(Pawn __instance)
- POSTFIX TheOcean.DWOverhaul: Void DwOverhaul.Pawn_Kill_Patch:Postfix(Pawn __instance)
- POSTFIX oskarpotocki.vfe.classical: Void VFEC.Senators.WorldComponent_Senators:Notify_PawnDied(Pawn __instance)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Log.Error_Patch1 (string)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Pawn.Kill_Patch11 (Verse.Pawn,System.Nullable`1<Verse.DamageInfo>,Verse.Hediff)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Pawn_HealthTracker.CheckForStateChange_Patch1 (Verse.Pawn_HealthTracker,System.Nullable`1<Verse.DamageInfo>,Verse.Hediff)
Verse.Pawn_AgeTracker:PostResolveLifeStageChange ()
Verse.Pawn_AgeTracker:AgeTickInterval (int)
Verse.Pawn:TickInterval (int)
Verse.Thing:DoTick ()
Verse.TickList:Tick ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.TickManager.DoSingleTick_Patch2 (Verse.TickManager)
Verse.TickManager:TickManagerUpdate ()
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Game.UpdatePlay_Patch1 (Verse.Game)
(wrapper dynamic-method) MonoMod.Utils.DynamicMethodDefinition:Verse.Root_Play.Update_Patch1 (Verse.Root_Play)

It's the technophobe trait making mechanoids immortal.

so its been over a month and still no fix? im still experiencing this starting a few days ago. any fix or work around outside of disabling the mod?