Default username and password are both admin. Did you change username or password?
If they are rejected, to login http://192.168.1.1, you may have to reset it to factory defaults.

M Diamond wrote

Default username and password are both admin. Did you change username or password?
If they are rejected, to login http://192.168.1.1, you may have to reset it to factory defaults.

Oh, I already know what happened. The password was too long!
It didn't work for 20 characters password, neither for 16 characters.
But it worked for 10 and 12 characters. Somewhere between must be the limit of password length.
Do you think I should report this as bug?
I use autogenerated passwords from KeePass, this is why it's so long.

Wow, how did you find it out? Good job!
What's the hardware version and firmware verison of your TD-W8970?
You can report bugs here. :)
http://ticket.tp-link.com/index.php?/Tickets/Submit

I'm a software developer, I'm just used to checking things this way ;]
I already submitted ticket to TP-Link.

Firmware Version:
0.6.0 2.12 v000c.0 Build 140613 Rel.31066n
Hardware Version:
TD-W8970 v1 00000000

You cannot login to telnet when password is longer than 15 characters.

It is a bug verified by TP-LINK support indeed. Actually, the password cannot be longer than 15 characters. If it is longer than 15 characters, only the first 15 will be saved as the password. When you login via web brower, it will only identify the first 15 as well and you can login. However, telnet still accept the complete password, so that you cannot login via telnet. But if you just type in the first 15, you will be login via telnet as well.
Not sure if I explained it clearly enough...

For me it is totally clear. However I think they should at least put information somewhere that you shouldn't set passwords longer than 15 characters.
Instead there will be user complaints about this issue.

I cannot agree with you more. :o