I just got that message too. Previously i have just made a backup project and then start to eliminate objects. First i removed all SObjects as they were the last imported objects. Then walls and objects, after that i deleted terrain, then all tracks one by one.. So i had 0 objects/meshes and still getting an error complaining about objects..
Then i found the culprit:
AIW !!! It was the same AIW that was exported just moments before with success so i really couldn't think that it was that.. I've had previously the same with cams, one cam was thrown so far that btb crashed when i tried to center to it (don't remeber how i solved that, i think i just managed to delete the cam fast enough
... Never had this with AIW, not to mention i didn't even touch it and there was no change in terrain or roads.. Weird.. I still have one faulty copy of the project if i want to analyze it further. I do think that it was one of the XGates bugging as they are objects too..
I have resolved this issue many times by simply editing any part of terrain, save and restart BTB. BTB did something funny just before i had that bug: i had one invisible SObject, i have seen this one many times with walls and SObjects. It happens when opening a project without restarting and only when i have edited wall/sobject in the previous session but it usually vanishes by itself if i click any other wall/sobject. That invisible wall/sobj can also be moved or deleted, the latter i usually use. This time the invisible sobj was excatly in the same location than grandstand sobject so i didn't notice it first, just wondered why the grandstand sobj didn't move when i dragged it's nodes.
I had also a faulty XPack at one point causing debugger to start communicating with me: I did housecleaning on that new XPack and erroneusly deleted one material path that the project still used. I don't think it was that issue here bugging as the project did export after i solved that.