I added a single record to firewall using UI and server stops responding to ANY requests, required manual reboot

After adding a single ALLOW record to the firewall, my server stopped responding to any requests.
ping 'my-ip' still worked, but it appeared to stop responding to any other requests, which makes me believe it’s a firewall issue.
I can’t SSH, access any of my hosted websites, nothing.
All I can do is go into OVH and issue a restart, then wait 5 minutes for it to spring back to life.

This happened before a few days ago when I was configuring the server using Hestia. I wasn’t sure exactly what caused it but it was probably editing the firewall list on that occasion as well.

Anyone else experienced this?
How can I fix it?

How did the rule look like?

name of service

That should be fine

I know.
Any idea why it completely messes up and I have to reboot the server?

No idea… have addd like that Manu rules on my own server and never experienced any issues