Also having this issue! I finally was able to upload the build after the validation 10/private api issue from earlier, but I can only log in for a few seconds at a time currently to connect. My build is there successfully which is good, but I can't actually do anything before it kicks me out again to just a login in page or a blank page as seen above.
EveningSun
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Interestingly, I was getting the validation 10 error until I updated my system to the latest macOS version. Only then did i start getting this error via email; "ITMS-90338: Non-public API usage" Theres been basically no changes to this app since the last successful upload, so I don't really know what it could be referring to.