Invoices not being generated

I've been using WHMCS for a few years now, and this issue has only just started to occur.

I noticed a couple of days ago that no new invoices were being generated when the cron runs. This is strange - almost every day we see 20-30 invoices being generated.

I thought this might have been a one off, but I noticed it again today! Something is definitely wrong.

My WHMCS is setup to generate invoices 14 days in advance. I confirmed the issue by checking whether customers who placed orders on the 1st of March have invoices - they don't.

No errors are being generated in my Activity Log. I followed WHMCS' suggested debugging strategy by enabling PHP errors and display errors, however the cron page never actually loads - so I can't see an error. This is because the backup never actually completes - a problem I've had for a while, but I'm fairly sure is unrelated.
My max execution time is set to something ridiculous, like 2000 seconds, and I've set memory_limit to 512 M just to be safe.

Does anyone have any ideas how this can start occurring? I think the only thing we've done recently is update the CentOS VPS the site is hosted on.

For any staff that are looking at this, I have a ticket open - 033902

Thanks guys :)