Unable to load workflow actions from the server. Please contact your server administrator

When you try to open the workflows in the SharePoint Designer, you receive the below error:

Symptoms: You can access the workflow from the SharePoint UI. Other workflows in different web applications are editable.

Cause: Cannot be same in all the scenarios but should be specific to the features

Capture a fiddler trace while opening the SharePoint designer and you should see a 505 error with the below error tag:

<errorstring xmlns="http://schemas.microsoft.com/sharepoint/soap/">Failed to find the XML file at location
     '14\Template\Features\custom solution\feature.xml'</errorstring><errorcode
     xmlns="http://schemas.microsoft.com/sharepoint/soap/">0x80131600</errorcode></detail></soap:Fault></soap:Body></soap:Envelope>

This is due to the feature which is not available at '14\Template\Features'. If you have a custom solution whose web parts are placed in a web site and the solution files are missing or the solution is retracted, you see this error.

Solution:  Remove the web parts that the solution adds to a site collection using PowerShell/code or deploy the solution back to the farm to get the feature.xml.