Using Office365, with applications installed on a MacBook Air, I cannot open attachments with Outlook for Mac 2011 Version 14.3.9 (131030).  Looking at the yellow triangle at the bottom right I see the message stating, "Unable to download the attachment in message ...<subject>... " with an "error code: -17890 showing at the bottom of the error screen.

These are not large files, and include PDFs and .doc(x) files primarily, but even .png files.

My mailbox is <500mb (limit 50gb), checked settings, ... not seeing an answer in forums...

Does anyone know why this would happen and what to do to fix it so I can get my attachments?