We're aware of some 503 errors affecting a small number of websites running caching plugins such as WP Super Cache, W3 Total Cache and WP Rocket.
We have traced this to an issue with a recent upgrade to our firewall and security systems powered by Imunify360.
As a temporary fix we have downgraded the affected components and all websites should be working normally.
We're continuing to work with our vendor for a permanent fix.