Best guess is that there is a conflict with Restrict Content Pro. They are enqueueing a fairly old version of jQuery Validation on the Connections admin pages. The version included in Connections is also out of date which I will correct in the next update. But that is not the issue.
Connections also uses jQuery Validation and is coded to a newer version. Their older version is overriding Connections. This conflict seems to be preventing the actual submission from taking place. When you click Add or Update, the page should refresh not do nothing. When you choose an image the validation fails due to the conflict and the image is never even uploaded.
To test my theory, Restrict Content Pro will temporarily have to deactivate it and then do a test by adding an image to an entry in Connections. Let me know how it goes!