During a subsequent test event (configuration details here), I tried to set stp and igmp snooping via luci for the one bridge defined and lost connectivity to the device. Using a serial cable to access the device, I could see that theses options were not enabled from the luci attempt. However, I was able to set these options in /etc/config/network and restart the network through the command line without issue.
The logs I saved around the time of this event don't show anything useful; however, another user observed a similar issue here.
I won't have an opportunity to test DSA/luci again until the weekend...
HTH