Outlook 2010 some sent items marked unread now (was Ok before)

Willy K

Outlook version
Outlook 2013 64 bit
Email Account
Last week my Outlook 2010 client suddenly started showing some of the sent items (emails) marked as unread. I've been using current configuration for years and this has never happened before.

My local folders are mapped to corresponding IMAP folders on provider's server (it's been like this for a long time), and I can't see any changes there or in my configuration. Everything else works fine.

I've tried to figure out this myself, but in vain. Microsoft posted recently a major Microsoft Office update, and I wonder if it could have anything to do with that.

Can anybody please help me out? Thanks!