How to find what causes error Blocked script execution in 'about:blank' (in desktop Safari)?

Question

I have noticed that when visiting my website from Safari (on macOS), the last entry in the console log is Blocked script execution in 'about:blank' because the document's frame is sandboxed and the 'allow-scripts' permission is not set. and the count of this message keeps increasing every second reaching a count of thousands (about 100 per second) until the computer runs out of RAM and the browser (or website tab) crashes. I suspect that it might be caused by an ad but how can I check?