Wow. We have been working on adding product to a client's website for several months and have experienced this issue MULTIPLE times. After contacting Squarespace Support on several occasions and being told it was our computer(s)/devices, our internet connection, cookies in our cache, etc, we finally found this thread. Not once did they suggest going back to the Classic Editor. Knowing this would have saved a tremendous amount of stress and extra work. Like a few of you have already said, if it ain't broke, don't try and fix it!