Hi Michelle,
Sorry you are still running into issues. I'm not running into export issues testing against the Map Services I have access to and have confirmed that BUG-000131306 has been fixed with the test service provided in the bug. I also tried with the following Map Service (https://landscape1.arcgis.com/arcgis/rest/services/USA_Roads/MapServer). I'm able to open the PDF result without error.
Here is my layout (using version 2.6):
The properties for the legend item (Map Service Sublayer "All roads") - removing the label and description and leaving only the label for the Group layer/Map Service Layer:
Here is the PDF output:
Here is the JPG output:
I did notice an issue where the legend on the layout page would "disappear" sometimes when selecting or unselecting the element.
This bad state did not affect the export result (PDF or JPG). The output displayed the legend correctly. I'm currently investigating this "bad" state issue.
Perhaps there is something specific with the Map Service you are using? IS this a public service that I might have access to?
Tom