Issue
When opening the Nintex Workflow Designer HTTP 500 errors occur accessing /_layouts/15/NintexWorkflow/Live/OnlineActivityServer.ashx?checkSource=1
Resolution
Update to the following build or newer:
Nintex Workflow 2010: Build 2.5.7.10
Nintex Workflow 2013: Build 3.3.1.20
Nintex Workflow 2016: Build 4.3.3.20
Nintex Workflow 2010: Build 2.5.7.10
Nintex Workflow 2013: Build 3.3.1.20
Nintex Workflow 2016: Build 4.3.3.20
Additional Information
This was a known bug in older versions of Nintex Workflows.