Hi JTeunis,
We have received several reports about this issue. The fix for Outlook for Mac 2011 has been released. For the issue on Outlook for Mac for Office 365, please keep an eye on the post below to get the latest updates.
http://community.office365.com/themes/groups/generic/forums/thread.aspx?ForumID=153&ForumThreadID=282030&GroupKeys=en-us/
Thanks for your understanding.
Best regards,
Ran