I can't think of anything off the top of my head and do not know of any bugs that would cause this.
I have a few questions and maybe the answers will give us a clue.
Is there anything in the Windows event log from either the ixn or ans (teaming) drivers?
Does the team have a fixed IP address or DHCP? When the team is not communicating does ipconfig show the expected IP address as assigned?
Is there anything in the switch log that might be related to communications issues?
Is the X520-T2 in each server installed in the same slot? Any differences at all between the servers that you can think of?
Which network connections software version is installed?