When I try to use "Google" button (i.e. Login with Google OAuth) then I get the following error message:

[phpBB Debug] PHP Warning: in file [ROOT]/vendor/lusitanian/oauth/src/OAuth/Common/Http/Client/AbstractClient.php on line 70: OAuth/Common/Http/Client/AbstractClient::OAuth/Common/Http/Client/{closure}(): Argument #2 ($key) must be passed by reference, value given
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 1813: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3027)

AbstractClient.php:70 looks like this: 

    public function normalizeHeaders(&$headers)
    {
        // Normalize headers
        array_walk(
            $headers,
            function (&$val, &$key) {
                $key = ucfirst(strtolower($key));
                $val = ucfirst(strtolower($key)) . ': ' . $val;
            }
        );
    }
User Contributed Notes may suggest the likely problem here, though I have run any tests myself:

"""
It's worth nothing that array_walk can not be used to change keys in the array.
The function may be defined as (&$value, $key) but not (&$value, &$key).
Even though PHP does not complain/warn, it does not modify the key.
"""

*https://www.php.net/manual/en/function.array-walk.php*

I'm can reproduce the issue with phpBB 3.3.10 (haven't tried other versions) with PHP 8.2 (also reproducible with PHP 8.1).

When I click on the "Google" button get the error page given above. I am not redirected to the Google Login page (at least not visibly). 

Notably, when I use the "Google" button with in an "Incognito Tab" then I do get the Google Login page. I have not been able to figure out the difference in state between "incognito tab" and "normal tab that previously used login/logout with a normal password".