Sometimes browsers may try to download old files from the browser cache and this may cause problems with cash register. This can happen after the upgrade, for example.
You can find the Refresh -button by first clicking on the menu button in the top menu of your browser. Marked with red in picture.
Next, click the Refresh -button at the top which will fix the situation.
Refresh -button may look slightly different depending on the device or browser you are using.