If you have an edit page open too long it will time out, failed token check. The best thing to do is copy what you have, paste into notepad or a word document and work on it there.
When you are ready, paste into the edit section and save. Then you can make changes, preview and save again.
If you do get the failed token check, what you have typed should still be there, copy it all, clear the biography box and paste it back in, then save.