this is weird... lets try one bypass before we actually look into it: on a terminal:
0. quit Evolution
1. tar -cvf savedmail.tar ~/.evolution/mail/local
2. cd ~/.evolution/mail/local
3. rm *.ev-summary
4. restart Evolution -- check if *all* emails are now shown
This will tell us if the summaries got corrupted (I do not think *all* of them would, but, just in case...