If you go into WwiseAudio folder in Unreal Editor and type into the following into the search:
ID=FEEB8BFF
It'll find any AkAudio Objects whose GUIDs start with that. Odds are, only one of them does. If more than one of them do, it won't be many, and it should be easy enough to find the one you're looking for.
Once you've found which object they're talking about, search for it in Wwise Project by name.
Right-click on the event, switch or rtpc in the project and select "Open containing folder" for the .wwu. Look for the specific object in Wwise.
Does the GUID in Wwise match up to the GUID in Unreal? If they don't match up, this is why it's saying that the Object ID is unknown.
If they do match up, I don't know what's going on.
Assuming they don't match up, you could delete the Event and do a force asset sync to recreate it.