I can not log in with the WinCustomise Browser either. It says "Error logging into your WinCustomize account!"
If it is no longer being supported, then why was I provided with a download link when I recently renewed my WinCustomize account? In fact, the download link was supplied to me again this week as part of a trouble response from Stardock Sales.
I found the WinCustomise Browser more convenient than my web browser. So, does/will it work or not?