Hi there. So this appears when the EKF "changes lanes". One of the large changes in AC3.4 vs AC3.3 is that there is one EKF per IMU (so two on a Pixhawk), in AC3.3 there was one EKF that used a blend of the two IMU's data.
That error means that it switched to the 2nd EKF. It'll do this if it gets too unhappy with some of the data coming in. When it does this, the vehicle tends to give a little motor spurt as well (something I just noticed and will fix in a future release).
In general it's not dangerous but it likely means some sensor data was not good. Paul Riseborough is probably the only person currently qualified to say whether it's a serious problem or not. You could post a dataflash log if you like and I'll ask if he can have a peek.
Thanks for the report. I'll update the wiki to let people know about this new error.