This error can occur in a variety of scenarios. Users should evaluate the issue and consider the following:

On-premise/locally hosted Raiser's Edge
Raiser's Edge hosted by Blackbaud:
When hosted, this message usually indicates that the user is at or close to their maximum storage space in the Content folder of the hosting environment. Please delete any unneeded folders and attempt to merge again.
For more information about the storage limit for the hosting environment, refer to What is the data size limit for Blackbaud Hosting Services files folder?

Error occurring in Raiser's Edge while using the Microsoft integration:
This error has been reported in conditional mail merges when rule statements such as IF, ELSEIF, NEXT, NEXTIF etc. are included. This is a limitation of Microsoft Word (documented in Microsoft's knowledgebase: Best practice when creating conditional mail merges in Raiser's Edge is to avoid the use of additional Microsoft conditional statements. To resolve this error users should remove the rule statements from the mail merge document in Raiser's Edge.