We tried the
7.0.2-1-r28 version of Liferay in sg region and we could sign-in to the Marketplace.
Did you do the same with the N. Virginia instance? Maybe this is the difference between the two instances behavior.
If you are ok with your new instance in N. Virginia region, that's great, but if you still want to have your instance in sg region, you can just try launching the instance again and let some time to the application to stabilize.
Hope it helps.