There are several bugs in the latest firmware 67.8 for all transceivers.
One of them is related to the data stream loss between transceiver and PC, which could lead to the abrupt stops of receiving process or straight to the "Lost connection with SunSDR2 PRO/DX!" error.
We know about these bugs, new versions of ExpertSDR3 0.9.X and ExpertSDR2 Update 10 will have fixed firmware.
Until then you can install the previous version of the firmware 66.8 and refuse firmware update if you use ExpertSDR3 0.9.0.