When the site is accessed via SPD from any other box besides the WFE/Workflow manager host server, the error is not encountered and its possible to save/publish workflows.

Reference: http://www.networksteve.com/enterprise/topic.php/Error_while_saving_a_workflow_via_sharepoint_designer:_Server-si/?TopicId=67706&Posts=1

When the site is accessed via SPD from any other box besides the WFE/Workflow manager host server, the error is not encountered and its possible to save/publish workflows.

• Reference Link.