Hii ...Im facing a problem with one of the controller AC800M
The problem seems to be in connectivity server or in a hardware. Engineering and operator stations doesn't have OPC servers so this is impossible that connection recovery after they are restarted (as per your text: After restarting the operating stations and Engineering stations the mill page is back to normal.)
Try to exchange Ethernet cables between 2 AC800M and check whether communication with another controller will fail or not.
Can you check that this controller IP is added in OPC panel in any other connectivity server. Our observation is that in case of single node machines, when we use same contoller ip at two different location that time, in one it will show yellow smiley in the other it will always show red. I hope this may help you to some extent.