Here you are, after updating to wordpress 3.5 you start getting this error because of a change in prepare() function wich now expects 2 parameters instead of one only.
The solution is to edit your plugin and change all the calls to prepare() in order to add a new dumb parameter, I used “NULL” for this purpose.
Or if you are in a search for RSS-digest plugin and if you are lazy, you can just Download an Up to date Version of rss-digest.zip.
I hope this will help others!!