"Error 404" when posting SignOutUser to Firebase

Question

I write a code to access Firebase. I use HTTPRequest to login, register, delete account etc. All work well but one method - logout = SignOutUser - always returns with 404 Error, unknown URL.

Links that work with HTTPRequest:

https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyPassword?key=%s
https://www.googleapis.com/identitytoolkit/v3/relyingparty/signupNewUser?key=%s
https://www.googleapis.com/identitytoolkit/v3/relyingparty/deleteAccount?key=%s

(with proper API key added to the string, of course)

And the one that doesn't work:

https://www.googleapis.com/identitytoolkit/v3/relyingparty/signOutUser?key=%s

What can be the problem? Can it be fixed? Is there any other way to logout a user? I write the code in gdscript, but I tried it via web code too, same result.

Thank you in advance!