Since the Sign Up link code is simulating an RFI hacking attempt which is being blocked by the BPS root htaccess file RFI security filter then a whitelist rule needs to be created to whitelist (allow) the register= Query String from being filtered.
1. Copy the code below to this BPS Root Custom Code text box: CUSTOM CODE PLUGIN/THEME SKIP/BYPASS RULES
2. Click the Save Root Custom Code button.
3. Go to the BPS Security Modes page, click the Create secure.htaccess File AutoMagic button, select the Activate Root Folder BulletProof Mode Radio button and click the Activate|Deactivate button.
# Clever Course Theme Sign Up/Register skip/bypass rule RewriteCond %{QUERY_STRING} register=(.*) [NC] RewriteRule . - [S=13]