If IPv6 is enabled it will prefer IPv6 DNS Lookups over IPv4 lookups. There are a lot of unexpected quirks when you use IPv6, so a partial roll out can cause this kind of issue. If you enable IPv6 and configure it to use IPv6 DNS servers then those devices could potentially take a completely different route to try to reach the destinations.
Have you got an assigned IPv6 routable address range? Such as a free tunnel from Hurricane Electric? If so, then carefully go through the network to make sure your devices are either all manually configured for their address, and the address of the DNS server, or that they get correct addresses from DHCP or SLAC. Also if you have IPv6 routing to the outside world make sure it is up and working.
There have been several mentions of 8.8.8.8 in this thread. That is a public DNS server that Google provides. they also have 8.8.4.4 and with IPv6 their public DNS servers are 2001:4860:4860::8888 and 2001:4860:4860::8844
Everything in this thread seems to indicate a partial rollout of IPv6, so it is OK to leave those devices disabled until IPv6 is fully implemented. Take the time necessary to make sure you have consistent configurations with IPv6. I hope that helps.