Since the latest update, all mails older than yesterday evening don’t open. I can see them, but no content is shown. Newer mails open properly.
This concerns my main folder only, the second folder works just fine.
An immediate solution is to uninstall eM Client, then download the version that was working from http://www.emclient.com/release-history
After installing the previous version, all your data should be as before.
Always make a backup (Menu > File > Backup) before uninstalling.