Can't get web scenario login to work

[ebonweaver]

I'm following the example document on creating web scenario but hit a wall at the login step. For some reason it seems to simply not work, but I can't figure out why. All it says is:
Error: response code "422" did not match any of the required status codes "200"
I can't even figure out what code 422 is, seems to almost never be used, I've never seen it before. Any ideas what's failing?

username={user}&password={pass}&login=Sign in

Based on the login page form showing:
input type="text" name="username" id="username-pulldown" tabindex="1" class="form--text-field"
input type="password" name="password" id="password-pulldown" tabindex="1" class="form--text-field -password"
input type="submit" name="login" id="login-pulldown" value="Sign in" class="button -highlight" tabindex="1"

[GPegel]

Resonse Code 422 means Unprocessable Entity. The request was well-formed but was unable to be followed due to semantic errors.

Question for you: In your scenario, which "Agent" is selected?

[ebonweaver]

Firefox Windows

[GPegel]

I use "Internet Explorer 10.0" as scenario Agent. Maybe you could try it also?

[ebonweaver]

I tried both IE 10 and 11 as well as Chrome, no change.
As you see from the code 422, it appears that Zabbix is not doing something in an expected way when passing the credentials to the login page.
It gives the same error code if I put valid or invalid credentials in the action, so it's clearly not an auth failure, Zabbix is somehow messing up the post action?