FYI, to make sure it is not a problem on my side, I got a new svnsyc'ed copy of
the full repository and tried this again.
It failed with exactly the same problem.
I'm getting an error about a non-existent URL, when trying to convert the large
ReactOS SVN Repository to Mercurial. See the attached log for more details.
I'm using the command:
hg convert -s svn --debug "file:///D|/Subversion/reactos" reactos
D:\Subversion\reactos is my svnsyc'ed copy of svn://svn.reactos.org/reactos.
In fact, the URL in the error message doesn't exist in revision 1904. It was
still "file:///D|/Subversion/reactos/trunk/freeldr/freeldr" at that time and
moved to the new URL in revision 12672.
Unfortunately, I also didn't find out, at which revision this problem started.
If I just convert with --rev=12672 or --rev=12673, the conversion runs successfully.