This happened to me on Thursday shooting wildlife. The viewfinder froze 3 times during the course of the day. Each time, turning the camera on and off did nothing and all other buttons were unresponsive. The only solution was to pop out the battery.
I had the grip on - all 3 batteries were the new 's' version. The SD card was a Sandisk Extreme Pro 280mb/s one and I was shooting with shutter priority, zone focus, CL set at 5fps and AF-C - I was switching between the different A-FC modes experimenting, so I can't say what I was on at the times that it happened. The lens was the 100-400mm.
Each time the viewfinder froze I noticed the image appeared to be a little overexposed.
There are a couple of threads over on DPR which say the same thing. Apparently Fuji are aware of the problem, so hopefully a fix is on the way.
I was shooting at the weekend without the grip and a different lens - 18-135mm. I wasn't shooting as extensively and only did a little AF-C, but there were no issues.