A quick poke around shows that the “admin” username still exists. I’m guessing you are actually able to log into the system, but when it redirects you to the dashboard, it freaks out on user permissions.
Now, I also tested hionline.nl/wp-login.php and hionline.nl/wp-admin and both are working correctly – so I don’t think the plugin renamed the wp directories.
I also installed the plugin on one of my installs and don’t have any problems with it (note: I haven’t let it change anything). I would see what happens when you manually deactivate the plugin by renaming the wp-content/plugins/wp-security-scan folder. When you ssh in, just drop into that dir then type ” mv wp-security-scan wp-security-scan-old” and WP will automatically deactivate it.