Not sure if you are facing the same problem I had. Tortured me for a long 3
weeks. Nothing I found online worked. Office Diagnostics could not help too.
Hope it works for you as it did for me. What a relief.
Mine was a case of corrupted PST/OST file. Repair using
scanpst.exe/scanost.exe that comes with Outlook may not work, so try renaming
your offline file, eg. "outlook.ost" to "outlook_old.ost", and let Outlook
builds a new one again the next time it starts up.
Good luck!