TortoiseSVN The coolest Interface to (Sub)Version Control

REPORT request failed on '...' REPORT of '...': 400 Bad Request (http://...)

You're behind a firewall which blocks DAV requests. Most firewalls do that.
Either ask your Administrator to change the firewall, or access the repository with https:// instead of http:// like in https://svn.collab.net/repos/svn/
That way you connect to the repository with SSL encryption, which firewalls can't interfere with (if they don't block the SSL port completely).