Ok that means you have invalid/bad code somewhere. Use FTP and delete your root htaccess file.
1. Copy this code below to this BPS Root Custom Code text box: CUSTOM CODE CUSTOM CODE REQUEST METHODS FILTERED
2. If you have added any other custom code in any other Custom Code text boxes then make a copy of it and delete it.
3. Click the Save Root Custom Code button.
4. Go to the Security Modes page and click the Create secure.htaccess File AutoMagic button, Select the Activate Root Folder BulletProof Mode Radio button and click the Activate|Deactivate button.
# REQUEST METHODS FILTERED
# If you want to allow HEAD Requests use BPS Custom Code and copy
# this entire REQUEST METHODS FILTERED section of code to this BPS Custom Code
# text box: CUSTOM CODE REQUEST METHODS FILTERED.
# See the CUSTOM CODE REQUEST METHODS FILTERED help text for additional steps.
RewriteCond %{REQUEST_METHOD} ^(TRACE|DELETE|TRACK|DEBUG) [NC]
RewriteRule ^(.*)$ - [F]
#RewriteCond %{REQUEST_METHOD} ^(HEAD) [NC]
#RewriteRule ^(.*)$ /wp-content/plugins/bulletproof-security/405.php [L]