I tried to switch and delete languages for my app in iTunes Connect. After deleting old screenshots it seems that I locked my self out. I guess the new buggy Media Manager refers to some old screenshots that do not exist anymore. 12.9 inch display says it uses screenshots from " 12.9 inch display", which makes no sense. So I guess the linked screenshots are gone and I can't do anything about it.
Is there any trick? I can not add any new primary language as workaround because it throws a screenshot error.
Edit: The first batch of screenshots was added before the media manager was introduced.