Ok, but I can’t undestand why only on Modx db !!
Sorry, but this is definitely a Plesk issue.
If you are using .htaccess to enable rewrite rules for friendly url’s, then this is likely a configuration conflict between the rewrite rules for MODx and whatever Apache craziness is configured by Plesk for PhpMyAdmin.
It’s the same!
I had a thought... what happens if you export to a csv file instead of sql?
Yes, but no result, always "?phpmyadmin=" added
And have you tried different compatibility modes?