So, this morning I did an ‘svn up’ on a project before I went to breakfast, then went off onto the other side of the hotel property to do some work and schmooze.
Later, I got this error:
$ svn up
svn: REPORT request failed on '/svn/reposname/!svn/vcc/default'
svn: REPORT of '/svn/reposname/!svn/vcc/default': 400 Bad Request (http://domain.com)
See, I hadn’t changed anything, and svn up on some other domains worked — which made the problem significantly harder to debug, actually.
Ultimately, the problem was simply a blocked port. Thanks to Daniel for figuring it out while I still had some hair remaining.