Symptoms
K2D4SP workflow (2010) is failing at the Create Folder step.
Server was unable to process request. ---> There is no Web named "/default.aspx/_vti_bin/K2SPDocumentLibraries.asmx".
Diagnoses
The SharePoint site URL was hard-coded at the Create step. It contained additional default value "/default.aspx//" and this caused the issue.
Sample (non-working) site URL: http://portal.denallix.com/sites/site1/list_name.default.aspx//
Sample (Working) site URL: http://portal.denallix.com/sites/site1/list_name
Resolution
Removing contained "/default.aspx//” resolved the issue.