It can be as simple as disabling the import option in LW's options, press apply, close preferences. Then open them again, enable importing & press apply. This is an issue that's not uncommon. However if this does not work, perhaps your LW preferences file needs deleting so it creates a fresh set. Updating to LW 4.18 did require new preferences for some people.

To be a step ahead just in case the first option does not work, then see Deleting LW Preferences folder in OSX. You should make a note of all settings before you do this however.