500 Internal Server Error

When attempting to log in to my page, I encountered a 500: Internal Server Error.

I have tried disabling all plugins and renaming .htaccess.

When I look at my host’s error log, I see multiple iterations of this (just with different timestamps):
20131019T110727: seareg.org/index.php
PHP Warning: Unknown: open(/var/php_sessions/sess_085ab28508e83370d4d1b01229f59150, O_RDWR) failed: No such file or directory (2) in Unknown on line 0
PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/php_sessions) in Unknown on line 0

I checked index.php and could not find any such call, unless it is somewhere in @var bool.

My theme is Twenty-Twelve (the same one it has been since the site was created). All plugins were updated the last time I was able to log in; I had deferred updating the WordPress version since I was mainly focused on updating content at the time.

According to my host, I am running PHP 7.3. The site was built several versions of PHP ago.

The closest I found on the forum was here: https://ww.wp.xz.cn/support/topic/php-warning-failed-to-write-session-data/, but the respondent indicated it was connected to a plugin.

However, disabling all plugins (https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/) did not fix the problem. I still received the 500 Internal Server Error when I tried to access the login page (seareg.org/wp-admin).

I am fairly new to this level of error in WordPress. Help, please.

The page I need help with: [log in to see the link]