Not sure if this will work but some ideas to try:
If at all possible first delete all of the nav mesh actors, and rebuild paths within UE.
If that doesn’t do it, I just posted something related to the disallowed object topic here that might help here too Asset Validation Issues - #14 by Hourences
If you copy paste the actors in the level to notepad, copying it again within Notepad, and then back to the editor that might sometimes strip out some of the data that is not longer present in that build of the engine.
Alternatively look for the nav mesh info within the text in Notepad and manually remove it there before pasting it back.