Constraints Prevent from Deleting Express Items

I recently took over the administration of an existing c5-site for a client that has some express entities and items. Trying to delete one of the items I get the following message:

An exception occurred while executing 'DELETE FROM ExpressEntityEntryAssociations WHERE id = ?' with params [97]: SQLSTATE[23000]: Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails (`db222645_167`.`_ExpressEntityAssociationSelectedEntries`, CONSTRAINT `FK_1F9C570CBF396750` FOREIGN KEY (`id`) REFERENCES `ExpressEntityEntryAssociations` (`id`))`

With PHPMyAdmin I found the mentioned table “_ExpressEntityAssociationSelectedEntries” and also the constraint preventing deletion. From the underscore I assumed it to be a deprecated table, I searched the code for it (v.5.8.4) and found it only in a “Version”-file in the migration folder with the following code:

    // Migrate data from old entries table to new
    if ($this->connection->tableExists('ExpressEntityAssociationSelectedEntries')) {
        $this->connection->transactional(function ($db) {
            $r = $db->query('select * from ExpressEntityAssociationSelectedEntries');
            while ($row = $r->fetchRow()) {
                $db->insert('ExpressEntityAssociationEntries', [
                    'association_id' => $row['id'], 'exEntryID' => $row['exSelectedEntryID']
            $this->connection->Execute('alter table ExpressEntityAssociationSelectedEntries rename _ExpressEntityAssociationSelectedEntries');

which as I understand replaces the table ExpressEntityAssociationSelectedEntries with a new one and renames the old one to _ExpressEntityAssociationSelectedEntries.

But why is it still there after the migration and why is it full of entries and why are there constraints? Does anyone know about this issue? Could the migration possibly been interrupted? Did the former admin do some strange hacking? Will I destroy anything if I delete the constraints or even the whole table manually? In 13 years I never saw any underscores in c5 databases.

Thanks in advance for any hint!