Steps to reproduce:

1. In the Backend > Go to Employee an edit a profile

2. Upload a photo

3. Error in console (s. attached screenshot)

4. Upload a pic is not possible

5. Error in console:

amelia-booking.js?ver=4.2:1 TypeError: this._addClass is not a function
    at e.<computed>.<computed>._create (sortable.min.js?ver=1.12.1:9)
    at e.<computed>.<computed>._create (jquery-ui.min.js?ver=5.8.1:6)
    at e.<computed>.<computed>._createWidget (jquery-ui.min.js?ver=5.8.1:6)
    at new e.<computed>.<computed> (jquery-ui.min.js?ver=5.8.1:6)
    at HTMLUListElement.<anonymous> (jquery-ui.min.js?ver=5.8.1:6)
    at Function.each (jquery.min.js?ver=3.6.0:2)
    at s.fn.init.each (jquery.min.js?ver=3.6.0:2)
    at s.fn.init.e.fn.<computed> [as sortable] (jquery-ui.min.js?ver=5.8.1:6)
    at n.initSortable (media-views.min.js?ver=5.8.1:2)
    at n.initialize (media-views.min.js?ver=5.8.1:2)

6. The error appears in all photo-uploads in the plugin, eg. Organisation, Services, Events, Emplyees

7. Tested with Chrome & Firefox on WP 5.8.1 with PHP 7.4.x and default theme twenty-twenty

Problem solved...

It was a conflict with another plugin, now it works fine