Please try the following in order until the issue is resolved.

If working with individual letters on specific records:
  1. Re-create the Letter code and the export associated with the Letter code
If working with mail merges under the Mail module:
  1. Re-create any query being used in the mail parameters
  2. Re-create the mail process and any letter used within the mail parameters (making sure to re-select merge fields)
If still experiencing the issue:

Click Chat with Support and reference this article.   Please be prepared to provide the following information to your Support Analyst:
  • The steps you were taking when this error occurred (step by step).
  • The Citrix username affected by this error.