Hmmph, I have the same problem as JonnyFiveAlive, and nothing seems to help.
I tried a lot of things and it still remains, making it about literally impossible to texture out my map...
So, if anyone can help, we would appreciate it.
Btw, I think it's layer related. When you save your session, it should save the data on texture layers as well, but it doesn't, so when you try texturing after restarting the Editor, it can't load that data and that's why this is happening.
But, I'm not sure.