Hmmm... Thanks guys. Okay, I can also reproduce it, but I don't know the fix yet. We didn't make any changes, but last weekend, the server moved to a new OS by the hosting company. I suspect it's related, but unfortunately, I don't know how easy to get around it.
It seems that even though I also get the error, what I tried to save or post still worked either the first time, or when I press the button the second time. So, try to do that. In the mean time, it is probably always a good idea to save the content you are about to post to a clipboard first just in case the post gets lost in this mess.
Sorry about the inconvenience.