3
This code comes from the file class/xoopssecurity.php.
If you're not using the default theme, try to change your current theme to the default in your database. The table to edit is (prefix)_config, column=conf_name -> theme_set and set the conf_value to 'default'.
Other option, try to login website_url/admin.php
Otherwise, report this as core bug and after this go back to 2.0.17 because 2.0.18 contains some sanitizing bugs.