Sounds like an IP conflict with the hmi since they clear up once the stratix is removed from the network. I would double check my IP addresses set in my stratix. The Stratix 5700 can be used as a simple dumb switch or as a master of a ring. Also has dhcp capability so you can make your network plug and play.
Are you using the 5700 with NAT capabilities or a regular 5700 with a separate NAT device?