I had a situation recently where we were trying to to configure K2 for SharePoin 2010 and it kept failing. The ULS logs turned up this error:
Application error when access /_admin/K2/K2SPCoreConfiguration.aspx, Error=Invalid URI: The URI scheme is not valid.
at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)
at SourceCode.SharePoint.ApplicationPages.WebControls.EnvironmentLibrarySection.PopulateEnvSPSiteUrlFields()
at SourceCode.SharePoint.ApplicationPages.WebControls.K2InputFormSectionControl.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint
The main thing of note in that error, and the giveaway of the problem is the stack of "System.Web.UI.Control.LoadRecursive()" that is the service repetitively trying to access a URL and failing and it is a URL defined by the K2 environment library in workspace for Sharpoint. In this case the problem was a poorly formed URL that had quotes around it but I have seen a similar error get thrown when there is no URL.