Close down Opera and delete sessions/autosave.win in the preferences folder. See "help -> about Opera for the location".
If that doesn't work, close down Opera and rename operaprefs.ini in that same preferences folder so that Opera creates a new one. If that fixes it, the problem is somewhere in your original operaprefs.ini. You can then just keep the new one and set any preferences you need to again. Or, you can restore your old one and do some trial and error (only edit the file while Opera is closed) to see if you can figure out what setting in the file is causing the problem.
If you have to, goto "All Messages/Drafts" and shift + del the draft.