Using FireFox I agree there is just a white page. But, using “developer tools” (f12) I see the error message:
[HTTP/1.1 403 Forbidden 1047ms]
This is exactly as described in the upgrade instructions. You have (at least) one custom file that should have been removed and was not, or that has not had “WT_WEBTREES” correctly changed to “WT_KIWITREES”.