SOLVED:
In the file: wp-content/plugins/wpdatatables/assets/css/admin/admin.css around line 378 you will find this:
.wpdt-c #adminmenuback {
z-index: 100;
}
Please, change form 100 to 1 and clare cache.
OR
Deactivate wpDataTables, than
You can install this plugin: https://wordpress.org/plugins/add-admin-css/
and put this in it:
.wpdt-c #adminmenuback {
z-index: 1 !important;
}