Here's the error in the C:\Program Files (x86)\Steam\steamapps\common\Motorsport Manager\MM_Data\output_log file:

ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
at System.Collections.Generic.List`1[Supplier].get_Item (Int32 index) [0x00000] in <filename unknown>:0
at TeamAIController.ChooseSuppliers (System.Collections.Generic.List`1[] inSuppliers) [0x00000] in <filename unknown>:0
at TeamAIController.FindSuppliersForNewChassis () [0x00000] in <filename unknown>:0
at TeamAIController.HandleCarNewChassis () [0x00000] in <filename unknown>:0
at TeamAIController.OnPreSeasonStart () [0x00000] in <filename unknown>:0
at Team.OnPreSeasonStart () [0x00000] in <filename unknown>:0
at Championship.OnPreSeasonStart () [0x00000] in <filename unknown>:0
at MMAction.Invoke () [0x00000] in <filename unknown>:0
at Calendar.DispatchEvent (.CalendarEvent_v1 inEvent) [0x00000] in <filename unknown>:0
at Calendar.DispatchDelayedEvents () [0x00000] in <filename unknown>:0
at Calendar.Update () [0x00000] in <filename unknown>:0
at GameTimer.UpdateSkip () [0x00000] in <filename unknown>:0
at GameTimer.Update () [0x00000] in <filename unknown>:0
at FrontendState.Update () [0x00000] in <filename unknown>:0
at GameStateManager.Update () [0x00000] in <filename unknown>:0
at App.Update () [0x00000] in <filename unknown>:0

Not sure if it helps or not. Seems like it's erroring trying to load the part suppliers maybe? Not sure how to fix it. Tried reloading, starting new career with the mod enabled and then loading this one. Didn't work.