Add media button doesn’t work after WP5.5 update

After the 5.5 update, the ‘Add media’ button doesn’t work any more in the classical editor. All plugins and themes are up to date. The site uses eco-press theme from CMSMasters with the CMSMasters Content Composer. The problem happens in all browsers.

I installed the ‘Enable jQuery Migrate Helper’ plugin, that doesn’t help.

Turning script debugging on didn’t help either.

The browser console displays the following error:

Uncaught ReferenceError: wpColorPickerL10n is not defined
    at h.<computed>.<computed>._create (wp-color-picker-alpha.js?ver=1.1.0:12)
    at h.<computed>.<computed>.h.isFunction.a.<computed> [as _create] (widget.min.js?ver=1.11.4:11)
    at h.<computed>.<computed>._createWidget (widget.min.js?ver=1.11.4:11)
    at new h.<computed>.<computed> (widget.min.js?ver=1.11.4:11)
    at HTMLInputElement.<anonymous> (widget.min.js?ver=1.11.4:11)
    at Function.each (jquery.js?ver=1.12.4-wp:2)
    at n.fn.init.each (jquery.js?ver=1.12.4-wp:2)
    at n.fn.init.h.fn.<computed> [as wpColorPicker] (widget.min.js?ver=1.11.4:11)
    at post.php?post=284062&action=edit:1782
    at HTMLDocument.<anonymous> (post.php?post=284062&action=edit:1782)

Now I’m stuck. Thanks to anyone who can help!