I suspect you've hit the .ost file size limit (50 GB) which a lot of people tend to run into because of OUC.

The fix is to turn off caching for shared folders and compact your data file (.ost)

  1. On the menu select Tools > Account settings.
  2. Click on your account, then on Change More Settings then select the Advanced tab.
  3. Clear the "Download shared folders (excludes mail folders)" checkbox.
  4. Exit and restart Outlook.
  5. In Outlook Click File > Account Settings >Account Settings.
  6. On the Data Files tab, click the data file that you want to compact, and then click Settings.
  7. Click the Advanced tab > Outlook Data File Settings.
  8. In the Outlook Data File Settings dialog, click Compact Now and then click OK.