I upgraded to version 2.5.12 in a local Docker environment and online. On the online side, the site works, but the panel is blank giving the following errors:
**Warning** : session_name(): Cannot change session name when headers already sent in **/usr/www/users/alexpa/sotiroudis.gr/kirby/vendor/getkirby/toolkit/lib/s.php** on line **48**
**Warning** : ini_set(): Headers already sent. You cannot change the session module's ini settings at this time in **/usr/www/users/alexpa/sotiroudis.gr/kirby/vendor/getkirby/toolkit/lib/s.php** on line **51**
**Warning** : ini_set(): Headers already sent. You cannot change the session module's ini settings at this time in **/usr/www/users/alexpa/sotiroudis.gr/kirby/vendor/getkirby/toolkit/lib/s.php** on line **52**
**Warning** : session_set_cookie_params(): Cannot change session cookie parameters when headers already sent in **/usr/www/users/alexpa/sotiroudis.gr/kirby/vendor/getkirby/toolkit/lib/s.php** on line **60**
That’s strange. The error message is a PHP error that generally only occurs if there was already output. Have you checked in the dev tools source view if there really was no output (not even a single character)?
Kirby 2 is currently not officially tested with PHP 7.3 as Kirby 2.5.12 was released before PHP 7.3 was available. But if you use the same Docker container locally and it works, that can’t really be it.
Issues like this are very hard to debug. One idea I have is that a file was corrupted while uploading/deploying. So what you could try is to replace the kirby and panel directories on the server.