Lightsail Internal Server Error 500 while trying to connect to our MSSQL Database over PHP

The HTTP 500 Internal Server Error is a general error message, meaning only that something unexpected happened on the server, and more specific information is not available. It could be caused by a number of things.

Here are a few more things to check:

• PHP Error Logs: Check the PHP error logs for more specific error messages. The error logs are often located in /var/log/apache2/error.log (for Apache) or /var/log/nginx/error.log (for Nginx), but the location can vary depending on your server setup.

• Database Connection: Test the database connection independently of your PHP application, to verify that the connection is working as expected. You can do this using a database management tool, or by running a simple script directly on your Lightsail instance.

• PHP Extensions: Make sure that the appropriate PHP extensions for MSSQL (like sqlsrv or pdo_sqlsrv) are installed and enabled. You can check the installed extensions using php -m command in the terminal or phpinfo() in a PHP script.

• Connection Strings: Make sure that your database connection strings are correct, including the database host, name, user, and password. Also make sure you're using the right driver in the connection string.

• Firewall Settings: Double-check your firewall settings, both on the Lightsail instance and on the server hosting the MSSQL database. Ensure that the appropriate ports (usually 1433 for MSSQL) are open.

• Database Server Logs: If possible, check the logs on the database server to see if there are any error messages there.

If you still can't figure out the problem, try creating a simple PHP script that connects to the database and runs a simple query. This might help to isolate the problem, by removing other factors that could be causing the error.