I am curious because I had one model export fine, then I simply replaced that model with another, renamed the base and the model to match, linked them, and now I’m getting “SanityCheck Failed” as an error and my model will not export. Says to look at a log for each model and Idk were that even is or if that will tell me anything. The setup for the new model is the same as the last as far as I can tell. Anybody have any ideas? 
I’m pretty sure it means that something is not welded (meaning that there is a hole in your model that will have no texture and it will look like a completely transparent spot even if there is mesh on the other side of it). If you can’t see the hole at all then it doesn’t matter. Failing a sanity check sounds bad, but it doesn’t cause any major problems that I know of. What program are you using? In GMax if you click the aurorabase (blue square at the bottom that is parent to your model) you can then click “view sanity check results” and it will tell you where to look to fix it.
Ah! thank you for this, was driving me mad! So far the issue seemed to be a texture name was too long. had to shorten it and everything then exported fine. I use 3DS Max 2020, with the NWMax and VelTools Plugins and it also has “View Sanity Check Results” so I just checked that.