Looks like any SVN repository requiring authentication has this problem.

E.g. my non-public work SVN repository (that is on the default port, over
https) has the same problem. However e.g. the public
https://mstreeview.svn.sourceforge.net/svnroot/mstreeview converts without a
hitch.

Both https://svn.cs.uu.nl:12443/repos/grauw-rep/ and my work SVN repository
use HTTP Basic authentication.