It seems a bug of the firmware and we have noticed that.
Now please follow the FAQ link below to register a TP-Link ID via Tether:
http://www.tp-link.com/en/faq-1366.html

Then you can login the router with the TP-Link ID via webpage.

I did register as a cloud user on the tether app. However, I still am only able to see the screen above. There is no option to login as the cloud user on the web interface.

According to Tech Support if the you did not change the use ID from "admin" then this bug will accept your current password. Otherwise it reports incorrect userID.

They say that a new release fixing this bug will be coming in at least one month.

I did not upgrade to 1.0.8 so I will wait for next release with bug fixed.

After registering my router with the cloud, I opened the web ui this am and now have a username / password prompt.

Similar post w/ solution: http://forum.tp-link.com/showthread.php?95018-AC5400-Unable-to-login-after-firmware-update

I did not want to bother with cloud so will wait for next firmware that fixes this bug...

Also, before automatically installing from router's web interface, check the support download page on TP-LINK website. They had already pulled the bad firmware even though it continued to show up on your router's admin advanced firmware page.

Tech support is supposed to fix it so the bad firmware does not show up on the router page. As of this morning it is still there.

How can I upgrade/downgrade when I can't login into my router :(The parental control dont work, the 2,4ghz is failing everyday, and it's impossible to logon.This router is a huge failure!!

How about only connecting one device to the router? if you are still unable to login, you may try a factory reset

How to fix the TP-Link = Buy Netgear.I got my money back, and I now have a Netgear ORBI mesh network.Working 100% .

I also like the above error, can not fix it.

I guess you can do a firmware upgrade after reset the router: