For anyone that uses Outlook 15 for Mac for Office 365, you might encounter the following issue: “An unknown error has occurred, the error code is: 0”.
There is a workaround found on the office 365 forum. I will be providing a slightly more detail step on getting about it.
As mentioned in the forum article, we would need to delete 3 folders in “GroupContainer” that have a random letter-number combination in the name and end with .ms .Office and .OfficeOsfWebHost. On OS X Yosemite, an efficient method to go to the folder would be to search for “Outlook 15 Profiles” using spotlight and going up 2 level in the folder to delete the 3 highlighted folders as shown below.
This seem to solve the unknown error for now, however do take down that after applying this workaround, you would need to re-sync all your emails in outlook as they are wiped off when you delete the 3 folders.