Same issue here. Attempting to edit any app info field doesn't work - and our existing app clip information has all been wiped.

This is happening to us too. The app store connect UI wiped all of our existing app clip configuration (image, subtitle, title), and trying to upload a new image to fix it doesn't work, nothing happens in the UI. Tapping 'subtitle' shows the same error as the original poster. It also blocks us from submitting a new build, since the UI complains about the app clip fields being required.

There is also another thread about this here: https://developer.apple.com/forums/thread/820074

I also went ahead and submitted a bug report here: FB22317839