Web DAV system folder 'X' is missing: This error indicates that the mailbox is missing essential system folders or references to system folders are missing.
Make sure the mailbox is not full. Exchange will disable access to the drafts folder if the mailbox is full. If necessary, clear content from the mailbox (examples: deleted items, junk mail, etc.).
Verify that all system folders are present: inbox, outbox, drafts, sent items, deleted items, junk email, calendar, contacts, tasks. To verify that a folder is indeed a system folder, see if it can be deleted in OWA. For example, if there is a folder called "outbox" and deletion is possible, then the folder is probably just a regular folder called "outbox" - not the outbox system folder. System folders cannot be deleted.
If you are missing system folders, consider running Outlook in mailbox repair mode.
- Open a command prompt,
- Navigate to the directory where OUTLOOK.EXE resides (for example, C:\Program Files\Microsoft Office\Office14).
- Run the command: OUTLOOK.EXE /CleanProfile /ResetFolderNames /ResetFolders
Once the operation has completed, resubmit your mailbox for migration.