'Fatal errors' causing Drupal website to crash

Hi. I've made a website for a friend using Drupal. Recently we've noticed that something is causing the site to crash, resulting in a 'fatal error' message. I understand that this can be caused by installing duplicate modules. However the error messages (see attachments) identifies the settings.php file and files in the 'includes' folder as being part of the problem. Given that they are part of core none of them have been altered by us, so I'm not sure quite what's going on here.

We've been getting around the problem by re-uploading the settings.php folder and the settings folder which gets things back on track but unfortunately the crashes seem to continue.

The Word doc attached shows the error messages we got on two separate days and the steps we took to get the site back up again.

I've used Drupal quite a bit but this is the first time I've come across this problem. Could it be in some ways caused by my friend's website hosting company? They don't have the best reputation.

If anyone has any ideas I'd be more than grateful!

Thanks