Eu resolvi o problema modificando a linha 864 no phpmyadmin / export.php, altere:
break 2;
By:
break;
Solução fornecida por bbalent na seguinte resposta :
Don't worry, this is a code mistake from the phpMyAdmin team: you are into one loop and not two nested loops. That's why you can't "break 2" (because 2 denotes that you're inside a nested loop). The error appears because PHP7 is stricter than former versions. By the way, this can fix other packages (PHPExcel, newer versions of phpMyAdmin etc.). Most of the time, the developper deleted the nested loop and forgot to update this line of code.