Existing workflows when opened through SharePoint Designer throw the following error : -
Steps to be taken : -
1) Ensure if you always get this error or if it is intermittent.
2) In case of the latter, try creating new workflow and see if any of the ECF custom actions are missing from the list.
3) If yes, try pointing to all WFEs (in case of load balancing) currently servicing that web application and see if the issue is replicated.