E-mails vanished for unknown reason. Even upon restoring the backup, they appear and dissapear right away!

This is a first time that this is happening.

For some unknown reason, emails before the 2 June 2016 disappeared.

I restored the weekly backup, when EM restarts and loads again, I see all the e-mails, and after some seconds, they disappear again!!

I have the files saving on a dropbox folder. I have also tried manually restoring all the files, but nothing.

Any help?