What happens when the meter is higher than 20??
Usually nothing bad happens IF the voltage is dropping when you insert your negative electrode. If the voltage does not drop from the reading when the electrode is not connected, then you can't be sure that the current regulator is actually functioning. It is quite normal and expected that the voltage will be higher on the 'High' setting.
Electrolyte does not go bad. Mine is over a year old and still works the same as when I mixed it.
My first thought is that you didn't use enough electrolyte. The electrolyte does three things. It keeps silver from plating onto the negative electrode which produces fine silver dust that will make your solution grayish. It provides conductivity which lowers the cell voltage. It activates the reducing agent.
With the right amount of electrolyte, you should be able to get a voltage between 10 and 20.
Also, if you are not using a stirrer, it would be a good idea to heat your water before you start.
Is there any way you could post a picture?