I just have this problem "solved" with help from basecam. I share it here in case more people encounter this problem.

The problem was that during yaw rotation, the camera started rolling to one or both sides, in my case sometimes up to 45 degrees. This became noticably after half a yaw revolution or more.

My setup was; 2 IMU sensors, with the frame IMU below the yaw axis. Everything was set correctly; IMU orientation, alignment with motor axis, advanced IMU calibration of both IMU's, etc, etc. Roll, pitch and yaw were correctly displayed by the three indicators in the GUI for both IMU's. Also the unwanted roll motion was indicated by the camera IMU.

The "solution" was that the frame IMU was defective, although the attitudes of the frame IMU that were displayed in the GUI correcsponded exactly with the actual frame attitudes.

Disabling the frame IMU solved the problem. So, at this moment my two IMU system is downgraded to one IMU, but at least the gimbal is functioning well now. I did not have to physically remove the frame IMU, although it is serially connected between the camera IMU and the controller board. This is convenient because I had my signal cable tailor made for the setup; shielded, grounded, guided and fixed in order to ensure a unobstructed movement of the gimbal and errorless data transmission.

Cheers.