Download of attached filed in incoming email severely delayed

Hi,

I have an annoying problem I would be grateful if someone could solve.

I run Outlook 2011 for Mac (14.4.4) which is synced towards Microsoft Exchange online. Everything works well except when there are attached files in incoming emails.  Sometimes it can take up to hours before an attached file is available (the email as such is available immediately), however luckily it is mostly done within 5-10 minutes. Meanwhile the notification bar switch between "Awaiting synchronisation for this folder" and "Downloading attached files...". There is no actual difference between larger and smaller files and there is also no problem sending emails with attached files, they are sent immediately.

I have tested in different locations with good connections to rule out unstable internet connection without success.

Thanks for any help with this issue!