This worked for me, thank you
Thank you very much. Is work for me too. Poland.
Worked for me ! Good Job 
@disco123 Your LG account is still locked. Please try unlocking your LG account again. Thank you.
The same problem with developer mode, sign in first
@troysev Please check my previous reply above and try unlocking your LG account. Thank you.
Same problem here in Finland. In https://developer.lge.com/apptest/retrieveApptestList.dev I can log in normally (no unlock screen), but logging in Web Developer App says “Try again. To proceed, visit LG developer site (developer.lge.com ) and sign in first”.
EDIT: Suddenly now worked! 
@narae0.kim
Hi, Narae0.Kim
for my user dok133do@gmail.com the issue with logging into the Dev Mode on LG TV also occurs. I went to the developer.lge…site and logged in, changed password there, but still cannot sign in, the same error occurs.
P.S. I have just tried the solution you posted above the other day, and it worked!
Thank you!
solved!
Restart the tv. That worked for me.