You transferred your websites to a new server and now your scripts are able to read the database, to display the data, but it can't write into the database tables. The main reason comes from the fact that the php version is different and a lot of function have been deprecated.
So you need to get the Mysql error to see what are the error message and determine the modifications that you need to apply to your codes.
To do so, you need to display the errors :
sudo nano /etc/php/7.4/apache2/php.ini
Set the following :
display_errors = On
display_startup_errors = On
error_reporting = E_ALL
Restart the Apache server.
The most common issue, is that you have forgotten some values in your "insert into" queries. It was not a big problem with the previous versions of php but now it can't send the requests properly to mysql.
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