Symptoms: Users find that in some instances of Internet Explorer, they are able to use the “Open with Explorer” button on the SharePoint List or Library tab. In other instances, the button is disabled. Closing and reopening the library sometimes corrects the problem.
Cause: The “Open with Explorer” button is not available in the 64-bit version of IE. When the problem is intermittent, it usually means that the client has both 64-bit and 32-bit versions of IE available on the same machine, perhaps with the shortcut to one on the task bar and a shortcut to the other on the start menu or desktop. The button availability changes based on which shortcut the user chooses.
Solution: Delete any shortcuts to the 64-bit version of IE. Replace them with shortcuts to the 32-bit version of IE. Note that with system generated shortcuts, the 64 bit version will be titled, “Internet Explorer (64-bit). If needed, search for “Internet Explorer” from the start menu to find both versions in the search results, right click the result which does not say “(64-bit)”, and choose “Pin to task bar” or “Pin to Start Menu”.