Thank you for your reply and your offer to help directly! I reactivated the old theme after seeing that the sidebar didn't work, so there's no hurry. I noticed that it says "blog sidebar", so maybe it just works on blog pages while I intend to use it on other ones. Could that be the reason? Even though I found it strange that all the widgets appeared in the preview, but not on the actual website after activation. I also noticed that in the pro version there is a "blog sidebar" and a "main sidebar". What is the difference between those two?