Hi,

I have got three Email accounts in my Outlook (currently 2016 preview, but the same problem exists in 2013):

- Private email account: outlook.com Address, configured as ActiveSync account
- Work email account: office365, configured as Exchange account
- University email account: office365, configured as Exchange account

The Problem now is that I can't configure the Data File (.ost) of the outlook.com account as default file.
Error Message: "The selected Outlook data file (.pst) format cannot be used. Outlook data files (.pst) must use the same format as offline Outlook data files (.ost). Select a matching Outlook data file (.pst) or turn off cache mode in the Account settings."

I have tried:

- removing all accounts and adding them again in different orders (didn't work in any way)
- disabling cache mode for all accounts as proposed

It's possible to set each of the exchange accounts data files as default, but not the one of the outlook.com ActiveSync account. :(

Set Another Data File as Default When Using an Exchange Account
http://www.slipstick.com/outlook/change-the-default-data-file-from-exchange-to-another-account/