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I used this link from your post: https://eontestnet.herokuapp.com/
The first screen was this:
first screen.png
the welcome screen, I clicked the x and went to the underlying screen:
2nd screen.png
Then I used the Login on top right hand side and got the regular login type screen:
3rd screen.png

Note the very subtle difference after I clicked login this screen showed:
4th screen.png

It basicaly stayed in that greyed out state and did not log me in. It just sat there, when I clicked the browser refresh I was taken directly back to screenshot 2 above. I tried it three times, and just now a forth for this response. No error message nothing about wrong key or anything, (I used my posting key as the password.

I'll try again in the morning if I have time.

Ok I'll try it soon that way to see if I can reproduce.

In the meantime, maybe try from https://eontestnet.herokuapp.com/login.html ?