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