wpsolutions on "[Plugin: All In One WP Security & Firewall] 500 error"

Have you enabled any other firewall rules in addition to the Blacklist IP/User agent rules?

Can you please check your .htacess file and show us contents for the blacklist IP/user agent rules. Just copy the content which appears in your .htaccess file between the IP and User Agent tags. See an example below:

#AIOWPS_IP_BLACKLIST_START
Order allow,deny
Allow from all
Deny from 188.190.98.156
Deny from 58.49.14.29
#AIOWPS_IP_BLACKLIST_END
#AIOWPS_USER_AGENT_BLACKLIST_START
<IfModule mod_rewrite.c>
RewriteEngine On

RewriteCond %{HTTP_USER_AGENT} ^SurveyBot [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^YottaShopping_Bot [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^baiduspider [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^webcrawl\.net [NC]
RewriteRule ^(.*)$ - [F,L]

</IfModule>
#AIOWPS_USER_AGENT_BLACKLIST_END