Do you get this error when updating the engine/project?
I usually get this error when I change/update to a new engine version and an asset feature was updated. The error usually involves a component or mesh that is part of the environment already gets updated and does not reset the same flag, so save all does not work, but states that you will get the same error each time until the level is resaved. The work around I use is to select window > levels and click on the save icon and it will resave the level even though save is not reset and the next time the level is loaded the error is cleared.