Salesforce echosign error: data value too large

Although I (Salesforce system administrator) am able to send an echosign document through Salesforce, when one of my users attepts to do the same, the following error appears:

Error Message: Error Message: data value too large: The call to the EchoSign web service call to send an interactive document returned an error message 'Unable to send on behalf of user: [Personal email address removed by moderator](apiActionId=CBJCHBCAABAAtxNbFKfrLGwPYYQZWVZK0lR2SukxR048)' and error code 'EXCEPTION' (max length=255)

Update failed. First exception on row 0 with id a0v380000076v5MAAQ; first error: STRING_TOO_LONG, Error Message: data value too large: The call to the EchoSign web service call to send an interactive document returned an error message 'Unable to send on behalf of user:[Personal email address removed by moderator](apiActionId=CBJCHBCAABAAtxNbFKfrLGwPYYQZWVZK0lR2SukxR048)' and error code 'EXCEPTION' (max length=255): [echosign_dev1__ErrorMessage__c]

What does this mean and how do I fix it?

TIA!