Mysql Remote IP blocked
Well, it took a couple of years of my life, but I think I've figured it out.

I had to increase two mysql database configuration values (in /etc/mysql/my.ini) :


max_connections (from 100 to 500 , for now)
max_connect_errors (from 10 to 10.000)


After that, I ran flush hosts on the mysql prompt to clear any blocked hosts.

The forum experiencing these issues is generating some heavy database queries (mainly statistics into a table with over 1 million records - not sure why this would be "difficult", but that's another topic). 

I assume this causes a temporary lockup and ultimately a lack of allowed connections. This in turn temporarily blacklists the webserver altogether!

After that, the database was accessible via the web interface again!