select2 Ajax error

Hey,

as in the sticky post mentionend, select2v3 was updatet to v4. I get the Error: Option ‘ajax’ is not allowed for Select2 so I switched off every other plugin (to check if it is a plugin conflict), but the error still shows up…
Getting it, when I try to add an attribute:

Uncaught Error: Option 'ajax' is not allowed for Select2 when attached to a <select> element.
    at String.<anonymous> (select2.min.js:21)
    at Function.each (load-scripts.php:2)
    at d.prepareOpts (select2.min.js:21)
    at d.prepareOpts (select2.min.js:22)
    at d.init (select2.min.js:21)
    at HTMLSelectElement.<anonymous> (select2.min.js:22)
    at Function.each (load-scripts.php:2)
    at a.fn.init.each (load-scripts.php:2)
    at a.fn.init.a.fn.select2 (select2.min.js:22)
    at HTMLSelectElement.<anonymous> (wc-enhanced-select.min.js:1)

So what should I do? It cannot be a conflict with another plugin…