Exceptions in workflow are not logged to the event log as you might hope but to a huge stack of text file logs in %Program Files%\Common Files\Microsoft Shared\web server extensions\12\LOGS.   These files contain all sorts of @#** an dto find the workflow exceptions you will need to search for  “workflow infrastructure” and “unexpected”

Workflow.  Here is a terrifying error I saw in my SharePoint logs: Engine RunWorkflow: System.IO.FileNotFoundException: Could not load file or assembly ‘System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified. Why the hell WF was looking for AJAX I don’t know and I don’t want to know. 

i just installed  “Microsoft ASP.NET 2.0 AJAX Extensions 1.0” now works