A quick checklist of possible solutions to what can be triggering the popup
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NoScript, Ghostery, Disconnect, Blur and Privacy Badger were reported by users to be causing this.
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Even if AdBlock is disabled or the site is whitelisted - it may be filtered through another adblocking extension. Try disabling all extensions and enable them back one by one to find what is causing this.
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Antivirus programs are sometimes blocking access to certain ressources. Avira Antivirus has been reported to trigger this popup. Disable temporarily your antivirus to see if it's causing the problem.
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Try launching your browser in safe mode. Here is how to enable it for FireFox.
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If you're using FireFox - in Tools / Options / Privacy / History - if "never remember history" is selected - this is also reported to be causing trouble.
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If you have modified your hosts file or if you have a firewall program - check them out and make sure they're not blocking access to Google's ads servers.
If nothing from the above helps -
get in touch and I'll help you fix it right away!
Sorry for the trouble ...