"The set of folders cannot be opened. The attempt to log on to Microsoft Exchange has failed." error message when you connect Public Folders in Outlook 2010. This issue specifically occurs after you upgrade Outlook 2007 to Outlook 2010.
To resolve this issue, apply the following update:
2584053 Description of the Outlook 2010 hotfix package (x86 Outlook-x-none.msp; x64 Outlook-x-none.msp): August 30, 2011
Note You must start Outlook 2010 after you apply the hotfix package. If you apply the hotfix package after you start Outlook 2010, the issue will still occur. To work around this issue, you must create a new profile in Outlook 2010.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.