If you are using water, check that the water is not already too warm. If the water is too warm, the S-bottle warmer will register an ‘error’ and you will hear a long beep. Ensure that the contact points of the heating element under the S-bottle and on the base are completely dry, and that no limescale has formed on the contact rings on the bottom of the element. If needed, you can clean the contact rings with a little bit of (cleaning) vinegar and the corner of a cloth.
Read more about how to clean the S-bottle warmer in the user guide (pdf) or watch the instructional video.