One thing I would check for is instances of file locking. We use various JSON and SBIN files to track sessions at times and if one or more of these gets locked by an antivirus, backup system, etc it could prevent us reading the current session details. A good utility for this is Process Monitor from Microsoft. You can configure it to look for any access to our install directory or the Domains folder (and subfolders) that isn't being initiated by MailService.exe, W3WP.exe, Explorer.exe, and Windows itself. That should tell you if anything else is touching and potentially locking those files. We have a fix coming in our next release which should make SmarterMail sessions handle these types of scenarios a little better, but file locking will always introduce trouble so its a good one to look for and eliminate!