@gathh - as explained in the comment I linked earlier I steamrolled through a similar error using the following with failing queries from the upgrade script:
SET foreign_key_checks = 0;
-- run some queries
SET foreign_key_checks = 1;
I fixed - hopefully… - another problem here by optimizing/fixing the table that was causing problems via PHPMyAdmin.
That issue apparently has something to do with using InnoDB vs MYISAM as engine and different charsets and collation. One has limits on the size of keys and that is causing all these foreign keys violations all over the place, I guess.
But no word on any of that from Mautic developers so far.