What is causing `input/output` errors when reading from NFS v4 on CentOS?

I found this searching for a solution to my own input/output error issues with a shared NFS mount. I was mounting a shared NFS drive on several machines, reading and writing with PHP. I was getting sporadic, but frequent, errors like this. I don't know if what I did fixed it, but on the off chance it helps someone else with the same problem ...

So, I was creating worker servers by cloning them. This resulted in them all having the same hostname. I didn't think anything of that, the hostname wasn't something that affected what I was doing, as far as I could tell. I change the hostnames to all be unique, and made sure the /etc/hosts file included the hostname pointing to 127.0.0.1, and the NFS errors haven't come back since.