Try these steps in order until the issue is resolved:

1. Exit and Sign Out of the Raisers Edge

2. If your export is a Blackbaud Conditional Word Merge try saving it as a Blackbaud Simple Word Merge
    a.  In your Export go to File > Save As
    b. Give the Export a new name and click Save (you should see the same export output fields from your original export)
    c. Click on 'Edit Merge Document and create a new letter (you can copy and paste the contents or type it in)
    d. Go to Add-Ins and pull down the Raisers Edge merge fields that you need
    e. Select 'Save and Return to RE7' when your document is completed

3.Recreate the export