Error on add 10 and more slides with pictures

Do you have WordFence installed by any chance?
(I’ve had this 403 error with the WF Firewall before).

Otherwise, have a look at your PHP “Max Input Vars” – that can sometimes be an issue and may require increasing – see: https://themes.artbees.net/docs/increase-max-input-vars/
(but this generally only becomes an issue if you have 50+ slides)