And as I suspected, the function needs to know to look at the form for the argument.
Combining WGM's fix for null, do it this way to get rid of the 3075 error:
=ConcatRelated("[Hive_ID]","[T_Hive_Inspection_Detail]","[Link_to_Log_Inspection_ID] = " & Nz([Forms]![F_Log_Inspection_Main].[Log_Inspection_ID],0),"[Hive_ID]",Chr(13)+Chr(10))