Open the problem model’s SKP.
Also open a new empty model SKP.
Switch everything on in the problem model [hidden geometry, layers etc, all-unlocked], Edit>Select-All and Edit>Copy
Go into the empty model and Edit>Paste-In-Place.
Save the ‘[now-not-]empty’ new model.
Also try doing a ‘Fix Problems’ on it etc…
Is that ‘copy’ OK ?
If so, close the problem version and replace its SKP with the new ‘copy’…
Well, that didn’t work - the copy had a lot of stuff missing.
However, it did identify the problem was an imported person in a wheelchair from the 3D warehouse. When I deleted that it saved with no problems, not even the need to “fix” anything.
So, yea…for now.
Did you try importing that 3D warehouse wheelchair into an empty model first? It’s always advisable to do that - sometimes just to clean up odd layering, and to check if the model is well structured with components. And maybe to check for errors of a more destructive kind!