Okay so further information it was indeed by another part of the code where the author also gave me a bypass to try out which seems to solve that error code as well.
1. Copy the modified REQUEST METHODS FILTERED htaccess code below to this BPS Root Custom Code text box: CUSTOM CODE REQUEST METHODS FILTERED.
2. Click the Save Root Custom Code button.
3. Go to the BPS Security Modes page and click the Root Folder BulletProof Mode Activate 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]That said however, the thumbnails are acting quite strangely as from time to time they seem to work for like a minute or two and then after that period they stop working all together. No error codes or alike but that sure is strange. I'm not sure if this is related to BPS either and more thinking about my CDN Cloudflare, but I'm not too sure about that one either.
Did you had any problems with that? Mainly I think it has something to do with the way it's loaded and uses query strings.