Possible that WinXP is not supported with KeeCloud?
A clean install of XP and then patched forward to .NET 4.0 produces a "an exception occurred during a webclient request" when attempting to connect to the url: "dropbox://filename.kbdx"
This same URL and username/password combo works on both Win7 and Win8. (This UID/pwd was generated with the URL wizard)
Is it possible that your code requires something in .NET 4.5, thus making WinXP an unsupported environment?