I’m experiencing this exact same issue. I have a Post Object ACF group on a page and when I drag and drop to rearrange the chosen posts this error (below) is thrown.
I do not “believe” its an issue with my theme — just using Underscores base theme.
I’m running the latest version of ACF Pro (5.9.5) and latest WordPress version (5.6.2)
Error msg:
acf-input.min.js?ver=5.9.5:3 Uncaught TypeError: Cannot read property ‘element’ of undefined
at HTMLLIElement.<anonymous> (acf-input.min.js?ver=5.9.5:3)
at Function.each (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,utils&ver=5.6.2:2)
at s.fn.init.each (load-scripts.php?c=1&load[chunk_0]=jquery-core,jquery-migrate,utils&ver=5.6.2:2)
at HTMLUListElement.stop (acf-input.min.js?ver=5.9.5:3)
at x.<computed>.<computed>._trigger (core.min.js?ver=1.12.1:116)
at x.<computed>.<computed>._trigger (sortable.min.js?ver=1.12.1:9)
at x.<computed>.<computed>.x.isFunction.r.<computed> [as _trigger] (core.min.js?ver=1.12.1:116)
at x.<computed>.<computed>._clear (sortable.min.js?ver=1.12.1:9)
at x.<computed>.<computed>.x.isFunction.r.<computed> [as _clear] (core.min.js?ver=1.12.1:116)
at x.<computed>.<computed>._mouseStop (sortable.min.js?ver=1.12.1:9)
