Rozemarijnte, every time you use the "Find errors" button, it deducts "gold coins" from your balance. (The quantity of coins you have earned appears beside your user name.)
My guess is that you may have used up your coins, so the "Find errors" function doesn't work until you solve puzzles and earn more coins.
If you click on the "Check" button, and the pop-up window doesn't appear, then it's likely that your pop-ups are being blocked. You can reset this in your browser properties. If that doesn't work, check the settings on your anti-virus.