I am having a similar problem.
I have successfully uploaded about 30 in-app purchase packages of Apple-hosted content using Application Loader, targeting an iOS app. However, once I added 5 iPad Pro screenshots to my iOS app description in iTunesConnect, I get 5 ITMS-3000 errors that reference these screenshots. Here is one example:
ERROR: ERROR ITMS-3000: "Line 107 column 97: value of attribute "display_target" is invalid; must be equal to "Mac", "iOS-3.5-in", "iOS-4-in", "iOS-4.7-in", "iOS-5.5-in", "iOS-Apple-Watch" or "iOS-iPad" at XPath/package/software/software_metadata/versions/version/locales/locale/software_screenshots/software_screenshot[10]"
I submitted a full report with screenshots and an Application Loader log file to the support people at iTunesConnect. After speaking with 3 different people during the course of nearly a week, I was directed to submit a support request to Apple Developer Technical Support.
I got a reply within hours from ADTS directing me to submit a bug report instead, which I have done today. I am now awaiting a reply.
From where I sit, this appears to be an error with Application Loader and iTunesConnect. Application Loader grabs most or all of the meta data from my app's record in iTunesConnect, downloads it, and adds it to a plist that becomes part of the package that is uploaded to iTunesConnect. I found one good hint for a workaround at StackOverFlow:
http://stackoverflow.com/questions/34275875/unable-to-upload-in-app-purchase-hosted-content-using-application-loader
I tried this once and successfully uploaded my in-app purchase content. The result was that my iPad Pro screenshots disappears from the account iTunesConnect. Once I restored the screenshots, the ITMS-3000 error came back.