After a day of debugging, in my case the problem was this line https://github.com/pimcore/admin-ui-classic-bundle/blob/v1.6.2/public/js/pimcore/asset/tree.js#L83 which got removed in version 1.6.3, which basically casts the id from number to string.

My pimcore version: v11.4.1

My solution was to downgrade pimcore/admin-ui-classic-bundle to version 1.6.2 in composer:

composer require pimcore/admin-ui-classic-bundle:1.6.2 --no-update

If you can go up in your version, here was this fixed: https://github.com/pimcore/admin-ui-classic-bundle/commit/34e6053b52a36bb143f8e87b43d5177fa8502dce