I can't find a more technically oriented forum to ask this in, so this will have to do. I'm using Outlook 2007. I am not using an exchange server. My PST file is locally stored. I have outlook open at all times when my computer is running, which is nearly 24/7. After unknown amount of time after a restart or after restarting outlook, this error message will invariably appear, if I try to access or any calendar events. Calendar is the only function of Outlook I am using. The file c:\users\(username)\appdata\local\microsoft\outlook\Outlook1.pst cannot be accessed because another workstation has modified it. Close and then restart all mail-enabled applications. You might need to restart your computer. It also seems reminders may not appear when Outlook is in this state, but no error appears until I click something. I also had Office 2010 beta installed, but just now removed it. Normally I am able to find any errors I encounter on Microsoft.com or elsewhere. However, this one produced very very minimal search results, all of which were related to network stored PSTs or Exchange environments, and usually involved older versions of Outlook. Thanks for any help.