Hi,
When using XML file data connections which need to be converted to udc files I constantely received "The solution has not been opened. The following error occured on opening: Value cannot be null. Parameter name: fileURL". This error happened in VisualStudio after converting a data connection in Infopath to an udc file resulting in an inability to edit the Infopath Form from the Visual Studio K2 anymore.
Looking up the Infopath manifest.xsf I found out that no fileURL for the XML data file was assigned by Infopath during the conversion process. The solution was to edit the Infopath manifest.xsf manually (see http://k2underground.com/forums/thread/21566.aspx for info how to do this). By adding the URL to the source (be sure to replace any & by &), I was able to use the Infopath form again without errors.