We had an issue where one of our customers could not synchronize any of his files on his laptop (the last time he tried was over three months ago). He continued to get an error saying “Access Denied” for each of the offline files he was trying to synchronize. After some troubleshooting I found that the error went away if the offline file encryption was turned off. Why was this causing the errors? Well after doing some more research I checked the group policy setting located under “Computer Configuration\Windows Settings\Security Settings\Public Key Policies\Encrypting File System” and sure enough, the certificate there had expired around 3 months ago. [more]
I had to generate a new certificate using the command “cipher /r:file_name” from the windows command prompt and then use the generated certificate to replace the expired one in the group policy. During the process of making some of these changes his offline file settings could not be changed, several of the buttons were grayed out and you could not select additional files to be used offline.
In order to fix this I had to clear the offline file cache. In order to do this in Windows Vista you have to create a new dword in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CSC\Parameters called FormatDatabase and set the value to 1, then reboot the computer. Afterward all of the offline file cache will be removed. This also fixed the problem with not being able to add new offline file items.