Drag.js (used for multiple file selection’s drag and drop mechanism) becomes too intensive so the browser becomes unresponsive. This can occur if you have made a custom publish layout for a member group prior to adding the new Assets field to the channel.
As long as the Assets field is visible in the DOM the problem wont occur. So the solution is to add it to the publish layout. Unfortunately the browser will become unresponsive before you have a chance to reset it.
To fix this you must go into the “exp_layout_publish” table in your database and delete the entry for the channel causing the problem.
The problem should now be solved.
Problems like this aren’t limited to Assets. It’s wise to leave the publish layout until the very end of the project and to consider these potential hiccups when planning future enhancements.