I have SharePoint Designer 2013 installed and working fine till I needed to install Visual Studio 2015 (I installed Enterprise but this error seems to occur with other versions). So I researched the issue where some say, you have to un-install then re-install in particular order.
Just run a repair on the SharePoint 2013 designer in control panel. If you still get the error, then check the cache folder in your user profile and make sure you have the following files:
replace the <yourname> and <sitename> with your information below.
c:\Users\<yourname>\AppData\Local\Microsoft\WebsiteCache\<sitename>\versionnumber\
Microsoft.SharePoint.WorkflowServices.Activities.Proxy.dll
Microsoft.Web.Design.Client.dll
Just run a repair on the SharePoint 2013 designer in control panel. If you still get the error, then check the cache folder in your user profile and make sure you have the following files:
replace the <yourname> and <sitename> with your information below.
c:\Users\<yourname>\AppData\Local\Microsoft\WebsiteCache\<sitename>\versionnumber\
Microsoft.SharePoint.WorkflowServices.Activities.Proxy.dll
Microsoft.Web.Design.Client.dll
