session handle lost

HI

I have a system developed in LabVIEW that controls at the same time different equipments: DMM, Fgen, Scope, PowerSupply and a Switch Matrix. The complete system works without problem, but sometimes, and I don't not why some instruments lost the session. For exaple, the last error was:

Error Code: -1074130544

Error Source: IviDmm Ivi Error Converter.vi

The session handle is not valid

The only one way to go over the error is closing the LabView and restart the system.... 

Why does this error happen?

How can I solve it withour stopping and restarting the whole system?

thanks