Same issue here. I'm unable to add screenshot for IAP right now - very frustrating and have wasted 30+ min already unable to get things added. Known working images just get stuck processing. Eventually it shows "Something went wrong. Delete the assets and try again." which doesn't work.

Apparently this is a recurring issue with reviewers or the process - not sure which. Either way, it's very frustrating to go through the very lengthly process of approval, finally have an app approved only to discover it can't be really be released when planned since the Subscriptions are stuck in Waiting for review or In review still. I've reported the issue via email contact form but doubtful it'll be resolved quickly given how slow all responses are recently.