From that error message,it sounds like something might be wrong with either the workspace or FME Server configuration.The macro that the error mentions is undefined (FME_SERVER_DEST_DIR) should be automatically set when the workspace is run through either the Data Download or Data Streaming services.
Just to confirm,has anything changed on your FME Server instance or within the workspace since this last worked?Also,do you get that error message with every workspace that is run on the system or just this one?One thing to try might be to see if you can run any of the workspaces from the Samples repository just to confirm if the error happens with those as well.