I have the same problem with a new Archer C2 AC900 v3. TP-Link support advised me to fill a complaint, but, since it looks like a software problem, I’m a bit reluctant to do it. Are there any plans to fix this issue in an upcoming version of firmware? (Though I can’t login through web, I can login through Tether app, which should make an upgrade possible). Alternatively, is there any alternative firmware that can be installed on this model without access to web admin panel?

I'm having exactly the same problem. Bought a completely new Archer C2 V3, plugged in my home network. It started working, but I can't access the web interface due to:

POST http://tplinkwifi.net/cgi-bin/luci/;stok=/login?form=login 500 (Internal Server Error)send @ jquery.min.js:4
ajax @ jquery.min.js:4
a.su.Proxy.write @ proxy.js:1
doSubmit @ form.js:1
e @ widget.js:1
submit @ form.js:1
e @ widget.js:1
M @ login.html:195
handler @ login.html:195
dispatch @ jquery.min.js:4
v.handle @ jquery.min.js:4

Thankfully tether somehow works, so I was able to change the administrator password, but that still doesn't help my problem when trying to disable NAT and similar things. I was going to accept the fate, but the default configuration is making the router fail once every two days or so and it's making me sad... Did anyone ever got this to work? I tried all the browsers I could think of, Chrome, Firefox, Opera, Vivaldi, Edge, on both Windows and Linux, both plugged and wifi, nothing ever worked.

after installing the original firmware again, the router has been working perfectly.

s.o: Linux Mint and Fedora
Browsers: Chrome and Firefox

I solved the problem. Had the same thing with the Archer C2. It works when the router is connected to the internet and crashes when it's not. This is super frustrating when you just bought a new router and it throws an error like this out of the box. Not everyone wants to use this product to connect to the internet. What an embarrassment...