After clicking the button open up gmail, click on the button in the upper right corner of the screen. Then, go to “See All Settings”.
From here scroll down to signature and create a new signature (You can also remove the old one, as it doesn’t work). Name the signature to identify it. Click in the open box and hit cmd + v (Mac) or ctrl + v (PC) to paste the new signature.
Now look imediatly below the signature box at “Signature Defaults”, set both to the new signature you named earlier.