I'm using IE9 on Windows 7 and have been getting this message several times every day for about a year now. I also get the "Bad Request" message almost every day.
I had several folders but I wasn't using any of them. When I saw this topic I deleted all the folders but it didn't make any difference - I still get the "edit was not saved" message several times every day. I have to constantly go back and re-do any changes - very frustrating.
It happens when I'm adding a new task and setting date and time on new and old tasks. In fact it happens whatever I'm doing.
We have not been able to reproduce this error, so our current hypothesis is that it has something to do with your network configuration. Perhaps you have a faulty firewall or router that is causing the internet connection to be flakey. This would explain why only 3 people are having the problem right now. We will keep investigating, however.