Reciving ngnix error page, but not using ngnix

For the last couple of months, there were few occasions when all our users were getting ngnix Internal Server Error 500. And that lasts for up to an hour. But the strange thing is that we are using only Apache, not ngnix.

We have DNS completely configured on Cloudflare. Our servers are on DigitalOcean.

At this point, I have no clue about where the problem could be because as I said ngnix is not used by our team anywhere.

Any advice or a hint would be very appreciated!