The Redis 8.10 update that was shipped tonight caused Redis on three Plesk servers of mine to fail this morning.
After some digging, I found that the working directory was set to the server root (default? not sure). This seems to have worked before, but wasn't working anymore.
Here's what I...
Woke up to 3 servers and 150+ domains sending out nothing but the 421 error.
Thanks for the fix provided here that got everything up and running again!