What do the logs show as the reason for the failure?

I had this issue with 4.8x and 4.9x.

Do you have dns servers programmed into 5.6.1?

What is the setting in 2.1.x about dns set to (can't remember where the setting is) ? .the setting is called something like multi dns?
This makes it use the dns servers in 5.2.13 and 5.6.1 and controls which order they are used in.