Basically what happens is that Facebook fetches the data from the server and then displays it to you. For whatever reason, Facebook gets the error and you don't.
Not sure if this is applicable to you, but I found a help page explaining how to troubleshoot a similar situation.
https://support.cloudflare.com/hc/en-us/articles/217720788-Why-doesn-t-my-site-display-correctly-when-sharing-to-Facebook-
Either you put it in the "Under Attack" mode, which probably uses very aggressive secret rules, or you made some kind of a firewall rule blocking Facebook's IPs. So you will need to locate anything like that and undo it.
You can also do a quick workaround by whitelisting "AS32934" in your IP firewall. If this solves the issue, do look in the aforementioned settings for the actual cause and remove the override if you're feeling paranoid enough.