Does it only happen when you try to change both, or can you change just text or the image? Does it work if you take out the image?
Also, if you have more than one browser on your computer, try to login with another one and do it there. They'd probably solve it if it's an authentication problem.
It sounds like it might be a bug with the forum software. There's not a lot I can do without knowing whether it's just you, and what exactly triggers it.