You should go to your Hudson server at the following URL:
http://<hudsonrul>/scm/SubversionSCM/enterCredential
This will allow you to specify a top-level SVN URL and appropriate credentials. In most cases, username/password authentication is enough.
Edit:
You should try accessing the SVN server from command line, to verify if this is an SVN server issue, or a Hudson issues.
On command line, type:
svn --no-auth-cache --username [yourusername] --password [yourpassword] info http://svn.server.url
Replace [yourusername] and [yourpassword] with your username and password, without the square brackets. Replace http://svn.server.url with address of your SVN server
You should get back basic SVN repo info, such as last changed dates, UUID, revision, etc.