I do not know what kind of problems you have on your site, but I can only advise you to remove the plugin.So, I wanted to tell you that I have already disabled all of the legacy hooks and I wasn't even using the forms (I use Gravity Forms instead).
I re-installed PHP 7.4 and tried updating, nope.. STILL doesn't work. I even removed the "Templates" folder from the "Uploads" directory and guess what.. ERROR 500.
PHP:[Wed Feb 21 13:38:05.508493 2024] [proxy_fcgi:error] [pid 746066:tid 140413914555968] [client 123.456.789.10:60644] AH01071: Got error 'PHP message: PHP Fatal error: Cannot redeclare mailster_freemius() (previously declared in /home/user/domains/domain.com/public_html/wp-content/plugins/mailster/mailster.php:110) in /home/user/domains/domain.com/public_html/wp-content/plugins/mailster/includes/freemius.php on line 17'
The nulling is just bad lately and I'm not sure why these aren't being tested in advance.
There may be an incompatibility with some plugin.
Remove unnecessary plugins.
Clean the site cache.
Clean and optimize the Site Database with the Advanced Database Cleaner PRO plugin.
Most likely, you have a lot of garbage accumulated on your site (in the database).
Then deactivate all plugins.
Install the Mailster plugin in a new way.
If everything is fine, activate all deactivated plugins in turn.
Check the functionality of the Mailster plugin every time. You may see the source of the problem.