When trying to open any workflow in SharePoint Designer, Error gets fired like:
Unable to load workflow actions from the server.
Check Farm Level Solutions:
Browse to Central administration ->Click System Settings ->Click Manage Farm Solutions and Then,
Check Site Collection Level Solutions:
Browse to Site collection ->Click Site Actions ->Site Settings ->Solutions (under Gallery).
Put both of the browser windows side-by-side and see if you have anything activated at both scopes, means there is any solution which is deployed at farm level and the same solution is also uploaded and activated to solutions gallery at site collection level. If you find this, deactivating the particular solution at site collection level should do the trick to open workflows in SharePoint Designer.