One of our users had '$$' as part of their password which was causing the release to fail with the below:

The svn command failed.
Command output:
svn: Commit failed (details follow):
svn: OPTIONS of 'http://myrepo/mysource': authorization failed: Could not authenticate to server: rejected Basic challenge (http://myrepo)

They had no problem committing directly to svn (through tortoise and eclipse) or browsing the directory with a browser.

Once their password was changed to have no '$'s in it, the release process worked normally.