Well about recovering your logs is almost impossivel unless you find the right pasword witch is useless because it whould take a very looooonnnnnnnnnng time to crack it even if you knew how the encrypton process works.
Now about that bug i think ive found a quick work-around:
In the Messenger Plus! config menu go to the "Privacy Protection>>Passwords" tab try cheking the "Ask for my password (can be changed in the password window)"
(i think you have to change your password for this option to become available but dont worry you can change your password to the default one that you use later).
I know its a little boring to have to put your password (if its a long one than is veeerrryyyy boring) every single time you start up MSN Messenger but at least you wont get you password changed from time to time and you wont have multiple logs for the same person in the same month...
By the away you should save the logs that you can open to a safe place and then clean up that folder (donīt delete the folder it self simple delete all files and folders inside it)before you try the above... better safe than sorry :-þ
oh! sorry by any missspelling but english is not my native language lol
Hope it helps... See ya!