I am testing upgrade to Drupal 9. I upgraded core and logged in and fixed a few issues with module deprecated code and then was presented with a Twig issue where replace syntax had changed. I changed a few views that had embedded twig syntax to rewrite some results and logged out.
On trying to log back in to continue with views amendments I found that the admin login (or any other user) would not work. The anon user home page just refreshed with no error messages. The username and password are correct.
I used drush to get a one time login which successfully logged me in but the problem persists. I get the following log message of type 'php':
RuntimeException: Failed to start the session because headers have already been sent by "/home/mark/lampstack-8.1.4-0/apache2/htdocs/pd-net-dev/vendor/symfony/http-foundation/Response.php" at line 1239. in Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() (line 152 of /home/mark/lampstack-8.1.4-0/apache2/htdocs/pd-net-dev/vendor/symfony/http-foundation/Session/Storage/NativeSessionStorage.php)
Any assistance in interpreting this and resolving the issue would be welcome
PHP version is 8.1.4
?>). That is the cause of that exception, in most of the cases.