Problem solved by modifying line 864 in phpmyadmin/export.php, change :
break 2;
By:
break;
Solution reference in this page https://superuser.com/questions/1111521/export-page-of-phpmyadmin-goes-blank-after-update-php5-to-php7-0
and actually the first reference is by bbalent answer https://github.com/phpmyadmin/phpmyadmin/issues/11912
"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."