The Fortbyte number 20 of the decryption challenge is special. Its location depends on the part you are in since it appears in the center of the first three storm circles. Wait until the safe area appears, go to the center of the circle and retrieve the Fortbyte.
If you miss your chance with the first circle and the second one is formed, just go to the center of the second circle: it works with the first three circles.
Tip: if you have difficulty seeing the Fortbyte, it is topped by a purple flash symbol.