"Yes, it's a bit slow, but workable today" I have disabled the Cloudflare but now re-enabled it. Cloudflare is used to make the site load faster.
Here is the reply from my hosting provider:
"I've read the thread you've linked to and I can confirm that this would not be caused by mod_cloudflare. This is not how that would function. mod_cloudflare simply allows you to see visitors' IP addresses instead of CloudFlare's IP addresses. It's just a header that queries their IP and replaces visits to your website with theirs.
I'll note however that I do see a lot of mod_security hits. This is the issue for your website.
You can solve this by either,
1: Disabling mod_security (I strongly advise against doing this). If you must do this, then enable Suhosin (php extension) as a backup security means.
2: Updating your website. Many of the exploits mod_security is protecting you against is likely due to out-dated software. Once you update, most of these issues will go away. If you're already updated to the latest, then it's highly likely that custom software is causing this.
The issue is with mod_security (which can be disabled, but strongly discouraged)."
I have updated the code of this site and disabled the mod_security. I guess the problem will not occur again.