Wow that is a lot of plugins mentioning/using Select2. If you had all of these disabled at the same time (except the two ACF ones), and the problem still appeared, we can assume the problem is elsewhere.
You can check to see which version of Select2 is being loaded in your admin area using your browsers inspector/developer tools. Below is a screenshot of what that looks like on my laptop using Google Chrome. Safari/Firefox/Edge all have similar features just laid out a bit differently.
https://cloudup.com/cRmYbzz55pK
In my test environment I see Select2 v 4.0.13 being loaded, and perhaps more importantly it is ‘select2.full.js’.
I’ve been googling/reading about the error you are getting, and it seems to be caused by trying to use the ‘dropdownCss’ argument when using a slimmed down/non-full featured version of Select2.
Looking through the source for Advanced Custom Fields Pro, it looks like it ships with both versions, but I can only see it ever attempting to load the ‘full’ version. So I am curious to hear what is being enqueued in your environment. If it isn’t the full version maybe I can reach out to ACF support and see under what circumstances that should happen.