I have had the same problem, I am not sure what is causing it or why. I have had some luck with save-as and the put a -1 behind how ever you name your drawing. once you have done this you should see for example xxxxx-1 at the top of Solidworks and everything should be ok. If the file is ok then do another save-as but without the -1. That's my work around, not sure if that is good or not. Let me know if that works or not.