If you are resubmitting (for example, after a previous rejection), it seems like this can be caused by adding additional in app purchases.
Our app binary was rejected, we uploaded a new one and while we were at it we added an additional in app purchase to the submission and some extra text to the app review information. All other in app purchases previously submitted remained "In Review" so we were literally just adding in another to the submission.
We got this same error when clicking submit (behind the scenes a 500 Internal Server Error verified using FireFox dev tools). We tried Safari, Chrome, Firefox on Mac and PC - all up-to-date. Same behaviour.
We tried removing the extra new in-app purchase (and cut down the extra review text a little) from the submission and it immediately went through without a hitch. YMMV, but worth checking.