There might be plenty of reasons for this issue. Try the following steps to prevent this from happening:
1. Disable or whitelist Self-Serve platform in the ad-blocking extensions or programs (e.g Adguard, uBlock, Ad-Block, Ad-Block Plus) and reload the page.
2. Try to disable 3d-party extensions (crawlers, snippers) and reload the page.
3. Clear your browser cache and cookies and reload the page.
4. Try any other browser.
If you're running Chrome version 76 browser or higher:
1. Click the notification icon in the address bar.
2. Choose 'Always allow ads on this site' to enable Self-Serve platform elements.
3. You can check whether the Self-Serve platform is whitelisted by Google Chrome by following this link: chrome://settings/content/ads?search=sites
4. https://ssp.clickadu.com should be on the 'allow' list to prevent the error messages.