*bump*
I've actually just experienced this problem too.
My blog was running perfectly fine last week, and I went to post a new entry today and received this error message. I am absolutely stumped as to what is going on!
I'm still on 2.611.
When I go to my general MT interface, there is nothing listed under Five Most Recent Entries.
The Five Most Recent Comments lists just that, the five most recent comments

I can access my previous entries through the links on the comment editing screen, as well as through the Category Listing screen, but NOT through Edit Entries.
I cannot export my entries.
I've contacted my host, in case it was them who screwed something up, ESPECIALLY because I've made no changes to the back end of the blog recently!
In the mean time, can anyone point me in the right direction for fixing this?