Offline errors generally fall under one of the issues described in the post linked above.
Is the sensor mounted to metal or near a lot of metal? If it is near the corner of the window try moving it along the frame away from the current spot.
If you test the sensor by moving it closer to the panel for a while and the offline issues do not stop it is likely a sensor/system issue. Generally you will see the offline errors stop when you move the sensor close for a while.
If you have another one of that model you can swap the locations to test.