Our email alerts are giving errors stating that they are connected to invalid datalists. When you go into the datalist you see that it is not available for feed alerts - even though it worked the day before or last week. I have also seen it where you change it back and then save it and it doesn't save your changes.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.