By default Outlook is storing Settings, Contacts, Calendar, Notes and Tasks in a standard data file. You can check in which file, at the menu FILE - ACCOUNT SETTINGS and the tab DATA FILES. See also at Microsoft Office Support Site
Data files of an IMAP Account couldn't be defined as default until Outlook 2010. Therefore Contacts, Calendar, Notes and Tasks were stored in a separate file.
Since Outlook 2013 it is possible to define even IMAP Accounts as default. This can cause loss of your Contacts, Calendar, Notes and Tasks, even when you applied it as first IMAP Account after a new installation.
Microsoft has changed the format of IMAP Accounts with the release of Outlook 2013 and formerly used pst data files are ignored now. Outlook is creating a new "volatile" ost file as data file for each IMAP Account instead.
In case you define this IMAP Account as default, all Contacts, Calendar, Notes and Tasks will be stored in this ost file as well. E.g., if you switch to your contacts, it is indicated by the label "(only this computer)".
As you probably know, ost files are temporary files and only contains an image of your E-mails as they are stored on the server of your E-mail provider. If this ost file is defective, you only need to delete it and Outlook will synchronize all E-mails again. But unfortunately not your Contacts, Calendar, Notes and Tasks, because they are not stored on the mail server and can't be synchronized. They are rather stored on your local computer. You will have the same problem, when you're trying to move to another E-mail provider.
Separate your local data (Contacts, Calendar, Notes and Tasks) from your IMAP E-Mails!
Now you can see your new data file on the left side at the top of the folder structure in Outlook. Outlook is storing your Contacts, Calendar, Notes and Tasks separated from your E-mails in a local data file in the future.
Now you only have to move your existing Contacts, Calendar, Notes and Tasks to the new file.
Repeat this procedure for your Contacts, Tasks and Notes as well.
If you switch to the appropriate section Contacts, Calendar, Notes and Tasks, all elements should be found in the particular folders, which are not labeled with "(only this computer)".
At the end you can also remove the corresponding folders from your IMAP Account.
From now on the moved sections will be stored separated from your IMAP Account.
And that is just fine!
Download the Outlook Backup Software