This error only occurs if you're using Cloudflare on your domain, and to fix this issue you will need to login into your Cloudflare account directly Cloudflare.com.
No worries if you have purchased Cloudflare from Hostinger, once you set it up here, you will receive an email with instructions on how you can also login to the original Cloudflare account itself.
Once you have logged in to your Cloudflare account you will need to follow these steps:
In the menu bar select the SSL/TLS square button
From there you will see a submenu, from where you should select Edge Certificates and your path should look like this:
3. Scroll down to the very bottom, you should find Disable Universal SSL button:
Now by default, Universal SSL is always enabled, and to fix this issue you will need to disable it, wait for a few minutes for it to reflect and then enable it again. So, in the end, you will see the same window as before:
4. After all of the steps are done, you can purge the cache from your Cloudflare account as well. Select Caching button from the menu and select the Purge Everything function here:
Once it's done, give it a few minutes and your site should be back up and working in no time!