I'm unable to open and edit SharePoint workflows in SharePoint Designer 2013. However, when I uninstalled Visual Studio 2015 Enterprise from the machine, I was able to open and edit workflows just fine. Has anyone else experienced this and knows of a possible solution so that I can have both programs on my machine?
As you described in the comment the error you might be getting because of cache issue.
You have to remove these cache files from 3 locations on your machine
%APPDATA%\Microsoft\Web Server Extensions\Cache
You can refer the below post for additional reference
You need to install Office Tools for Visual Studio 2015.