Hi, Folks!
After migrations of mailbox 2010 -> 2013 at some point To-Do List stopped working.
Other migrated mailboxes do not have this problem.
On one mailbox To-Do List is empty. It is empty in Outlook. It is empty in OWA. It is empty on different computers when problem user log's in.
When you type in a task in To-Do List, it's been created in Tasks folder, however To-Do List stays empty.
How do I fix it?
UPD: while experimenting I figured out that if I enable Cached Exchange Mode, To-Do List works. Disable => To-Do List won't work. Other users do not experience this problem either way - with Cached Mode On or Off. Also there is no Cached Mode in OWA.