When trying to load a SharePoint 2010 workflow using SharePoint Designer 2010, an error message appears: "Unable to load workflow actions from the server. Please contact your server administrator."
A feature was missing on some of the SharePoint servers.
I ran a fiddler trace to find out what the issue was. I got a code 74 in Fiddler, then saw what it was looking for.
Then, I logged on the each of our SharePoint servers and found that on half of them (I have 6), the Powerview folder was missing from the C:\Program Files\CommonFiles\Microsoft Shared\Web Server Extensions\14\Template\Features folder. I copied the entire Powerview folder from a server that had it to the servers that didn't. Then, I did an IIS reset on all servers in the farm (although I suspect it would only need to be done on the servers where the files were missing).
I re-opened SPD and opened the workflow without issues.