Hi, thanks for your reply.
Unfortunately scanpst did not solve the problem.
Following your other suggestion, I have moved the local archive to the mailbox, and now the missing emails can be seen through OWA. They're still not showing un in outlook, though.
We are using Outlook 2010 with O365 hosted exchange.
There's a user that manually archives incoming email to a local archive folder (despite my suggestion not to do so).
Today she found that emails from a specific dates interval (approx. from end of january to beginning of march...