I recently switched operation mode from Access Point to Router (after having my cable modem set to bridge mode by the ISP).

Now, the administrative interface at http://tplinkwifi.net only works for a few hours after each router reboot. This was never a problem before, but has been a consistent behavior ever since the change to Router-mode.

When I browse to http://tplinkwifi.net (or 192.168.0.1) I am greeted with the router login screen.

If I enter an incorrect username/password combination, I'm shown an "incorrect username or password" error. So the page is accessing the router, and actively getting rejected.

But, if I enter correct/valid credentials, the page just hangs. It never leaves the login-prompt. After 30 seconds or so, the "Login" button re-enables, and I can try again, but login never succeeds no matter how many times I try it.

Only rebooting the router (turn the power off/on) works. Login to admin interface is always fine for a few hours after rebooting.

I've sniffed the traffic from my PC to the router during login, and can see that the login-attempt is a 

POST to /cgi-bin/luci/;stok=/login?form=cloud_login

This POST never gets a response (times out) when login fails. But with a "freshly rebooted" router, it responds correctly. So it seems the login-service inside the router crashes after some time. But (in my case at least) only in Router mode, not in Access-point mode.