I just experienced this error after I set up a new SVN client installation:
I know others have experienced it too, but I could not find a solution on these fora. I did some digging and eventually found that it was due to the presence of a proxy server, in my case Squid. I adjusted Squid's configuration by adding "REPORT MERGE MKACTIVITY CHECKOUT" to "extension_methods" and all was well again!svn: Commit failed (details follow):
svn: MKACTIVITY of '/svn/repos/joomla/!svn/act/20b34b89-e624-0410-bfb9-b7cf8f28e335': 400 Bad Request (http://scm.joomla.org)
Reference: http://www.wlug.org.nz/SquidNotes
Hope this helps someone else facing the same (unfriendly) error message.
Regards,
Chris.