One additional tip here for anyone who encounters this issue - If the log file does not point out what actor is causing the issue you could try following too:
In the viewport of the editor Ctrl A (select all), Ctrl C (copy), and then in Notepad or equivalent text editor paste it.
Actors placed in a level in Unreal convert over to a text format when you do so. And the text can reveal information that might otherwise not be exposed.
In this example following disallowed type was reported
If within Notepad we’d then look for “melee” we’d find this
Scrolling up (pay attention to the indentation) we’d eventually find the actor the object belongs to - the actor is the one that is shown to be in your level in the editor
If there is too much in the level to copy paste consider copy pasting a bunch of actors at a time until you find the refence.