I have setup a repo on my small server which i access via a sub domain "svn.site.com"

My vhost.conf has

#/var/www/vhosts/isuckatwebdesign.com/subdomains/svn/conf/vhost.conf
ServerName svn.isuckatwebdesign.com
<Location />
#svn.isuckatwebdesign.com -- vhost.conf file

DAV svn
SVNPath /var/www/vhosts/isuckatwebdesign.com/subdomains/svn/svnrepo/isuk
#AuthType Basic
#AuthName "Subversion Repository"
#AuthUserFile /etc/subversion/passwd
#Require valid-user
</Location>

After I comment the Authorization like i've within the code block I do not obtain a 500 internal server error when logging in to the repo also it shows the repo however i can't invest in the repo, whether it's uncommented it provides the server error.

Apache Error log shows

    [Sun Apr 10 07:38:55 2011] [crit] [client 214.27.58.2] configuration error:  couldn't check access.  No groups file?: /
    [Sun Apr 10 07:38:55 2011] [crit] [client 214.27.58.2] configuration error:  couldn't check access.  No groups file?: /error_docs/internal_server_error.html

In subversion you are able to define categories of customers and grant rights to individuals groups according to their authentication. Thus, in case your subversion repository has groups defined, and as you are not supplying any authentication whatsoever SVN cannot link your user to the group and you'll happen to be past due to some read-only group.

Are you able to make sure the SVN groups defined inside your repository?

I am unable to be entirely sure, however the book informs relating to this configuration options http://svnbook.red-bean.com/en/1.5/svn.serverconfig.svnserve.html#svn.serverconfig.svnserve.auth

I set up a repo several weeks ago and that i use Apache authentication and SVN groups.