|I would suggest you start with the Netgear documentation to reveiw what conditions Open Fail occures under.|
Though you have made what appears to be a logical connection between the sonicwall hookup and the open fail condition [I don't even know what that means] you have a Netgear GS724T between it and these FS726T switches.
You don't mention the GS724T having any issues.
I have to wonder if some how with your hookup you are creating a loop in the switched system. Now that would cause a fault condition especially if spanning tree protocol is engaged on the switches. They would shutdown the loop.