Maybe it's better to remove ISPconfig instalation and reinstall again?
Your problem is either phpmyadmin or wordpress according to your logs and both software packages are neither part of ispconfig nor belong to it. So reinstalling ispconfig does not make much sense in my opinion and it wont change anything regarding your problem.
First you will have to find out which software is causing the problem and my recommendation for that is to protect phpmyadmin with a htaccess password protection and then enable your haproxy again to see if the problem is fixed. The installation directory of phpmyadmin differs for every Linux distribution, for Debian and ubuntu it is e.g. /usr/share/phpmyadmin
Most likely a full rennstall is not nescessary, your problem looks more like the typical spam bot. So before you try a wipeout the server I would close the access to phpmyadmin and see if its fixed then.