using a Sharepoint 2013 VisualStudio Workflow with initiation form I wonder why a member of the site can work on the library elements and trigger the workflow but when the workflow tries to show its initiation form only the error page appears with

sorry, you don't have access to this page

? what might be the problem here ? the workflow has got its InitiationUrl in the Elements.xml - do I have to set permissions to the initiation form somehow?

or has it got to do with this: manually-start-sharepoint-2010-workflow-in-sharepoint-2013-farm ?