Today I encountered a new error at work which is:
“Unable to load workflow actions from the server. Please contact your server administrator.”
This occurred when I opened a SharePoint site in Microsoft Office SharePoint Designer, and then tried to add a Workflow.
Apparently, if one makes a mistake when deploying his/her custom activities, no workflows work.
The solution to this is to find the error in Visual Studio and then restart the IIS and try over again. Most probably, the error is in the new XML you wrote in the .ACTIONS file, so make sure to first check that out.
Goodluck!