NodeJS make problems under ELB

we are using ELB to Load balance between two NODEJS server.

Suddenly yesterday the service has started to recieve errors while i have two servers under the ELB.

when removing one of the servers and staying with only one server the service is working fine.

i don't have any log of traffic direction between the the servers and it seems that the system works fine with one server (no matter which one of them) and doesn't work with more than one server.

Any suggestions what should we check ?

10x!