If it is one of those lovely TP-Link bugs, then all bets are off. However, try some basic troubleshooting first:
Disconnect the WAN-side cable (phone line or ethernet, assuming it is either of these).
Also applies to LAN-side access: Make sure you access it with ethernet cable and isolate the router to just the one ethernet cable and client PC that you're trying to access the config page with.
Do all this before booting the router to make sure the boot is 'clean'.
Do you see the same issue?
If not, it suggests that something is interfering when other connections (WAN, LAN, WiFi) are plugged-in/active.
This may be at worst, a specific attacker, or otherwise anything of: some buggy process on any connected device, or some network connection type (overloading, bug, etc).
As suggested, the router running out of RAM (and a bug in handling this condition) is a common problem, especially under heavy load.
Also, check that there isn't some sort of firewall setting on the router (or even your PC) blocking your client IP from accessing the web interface. Some routers support this.
I would strongly suggest to always disable uPnP as mligor has suggested.
It is way too much of a security risk and if you need to open ports instead of uPnP's automated method, do it manually and learn what this entails.
Also, disable remote administration and set a strong admin password.