Logs at game start which mods/maps try to register custom fill types (mod name, count, status) – regardless of whether registration actually succeeded.
If the game’s internal 255 fill type limit is exceeded, a popup shows the full list of all mod/map fill types with their status (OK/Overwritten/Error). Individual fill types can be disabled directly in the popup to get back under the limit; the selection is saved to the savegame and takes effect after the next load.
The popup can be reopened anytime via hotkey (Shift+F, customizable) or console command, and an in-game help page explains all the details.
