This problem has not been reported with the Z1 before. However, the firmware may have changed.
Check if you have an EOFException on the frame and then delay 0.5 second if there as an EOFException and run the getLivePreview command again.
The frame format is motionJPEG, described in the document below.
theta_sc2_get_live_preview_2020_08_24.pdf - Google Drive
If you are not seeing an EOFException in the debug console, post again.
Another reference to the freezing frame
Question about getLivePreview by THETA SC2 on Android - #15 by tamy