I also attempted restoring the modified files, even though the modifications are minimal, the issue was still present. I recorded a video with the issue as I have to restore the modifications after testing.
The modified contents are as follows. They should not interfere with the sign up flow:
/oauth/authorize/index.php - Comment out 93-98 & 100
if ( $redirectUri )
{
//if ( !\in_array( $redirectUri, $allowedRedirectUris ) )
//{
// throw new \IPS\Login\Handler\OAuth2\InitException('oauth_err_invalid_redirect_uri');
//}
//else
//{
$obj->redirectUri = \IPS\Http\Url::external( $redirectUri );
//}
}
/oauth/token/index.php 312 - 316
/* Check we are not IP banned */
//$ipBanned = \IPS\Request::i()->ipAddressIsBanned();
//if ( $ipBanned )
//{
// throw new \IPS\Login\Handler\OAuth2\Exception( 'invalid_client', "IP Address banned" );
//}
/system/Dispatcher/Api.php comment out line 74
/* Check our IP address isn't banned */
//$this->_checkIpAddressIsAllowed();