Try each of the following steps in order until the issue is resolved:
 
  1. Disable and re-enable the Outlook integration:
    1. Disable the Outlook integration.
    2. Exit and sign out of The Raiser's Edge and Outlook.
    3. Reboot the workstation.
    4. Open Outlook, and re-enable the checkbox that was unmarked in step 1a.
    5. Edit the action again.
  2.  Be sure to check with your IT department before performing the following steps to ensure there are no ramifications from doing this.
    1. In Outlook, select View, Folder List from the menu bar.
    2. Highlight the Calendar folder and select View, Current View, By Category from the menu bar.
    3. Click the folded page icon in column header to sort the records By Icon.
    4. Click the [+] sign next to the Category heading and scroll down until you find one or more items with a crossed swords icon.
    5. Double-click on the item with crossed-swords
    6. From the Conflict Resolution window select to keep the item with the most recent date
    7. Synchronize

Alternative solution:
Delete the action from the workstation that does not have Outlook installed.