To fix this issue do these steps below:
1. Go to the BPS htaccess File Editor tab page.
2. Click the Your Current Root htaccess File tab.
3. Copy the BPS PRO ERROR LOGGING AND TRACKING htaccess code in your root htaccess file (example code is shown below – do not use the example code below) and paste it into this BPS Root Custom Code text box: CUSTOM CODE ERROR LOGGING AND TRACKING
4. Comment out this line of code with a # sign: #ErrorDocument 401 default or add a slash: ErrorDocument 401 /default
5. Click the Save Root Custom Code button.
6. Go to the Security Modes page and click the Root Folder BulletProof Mode Activate button.
# BPS PRO ERROR LOGGING AND TRACKING
# Use BPS Custom Code to modify/edit/change this code and to save it permanently.
...
...
...
# You can open the BPS Pro 404.php file using the WP Plugins Editor or by using the BPS Pro File Manager.
# NOTE: By default WordPress automatically looks in your Theme's folder for a 404.php Theme template file.
ErrorDocument 400 /wp-content/plugins/bulletproof-security/400.php
#ErrorDocument 401 default or ErrorDocument 401 /default
ErrorDocument 403 /wp-content/plugins/bulletproof-security/403.php
ErrorDocument 404 /404.php
ErrorDocument 405 /wp-content/plugins/bulletproof-security/405.php
ErrorDocument 410 /wp-content/plugins/bulletproof-security/410.php
Related forum topic: https://wordpress.org/support/topic/unsupported-errordocument-url?replies=7
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