Hi,
Please check if the following KB article applies to your situation:
https://support.microsoft.com/en-us/kb/2786607
Since this forum is for general questions and feedback related to Outlook, and your question is more related to public folder, I'd recommend you post a new question in the Exchange forum for further assistance:
https://social.technet.microsoft.com/Forums/office/en-us/home?category=exchangeserver
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Steve Fan
TechNet Community Support