That's not the URL of the button that is generating the popup. Right-click the button and choose inspect and look at the 'href' value in the button.
If you have 3rd party addons installed, suggest you disable them and see if the problem continues. If so, contact helpdesk. If not, and things are okay, start re-enabling the addons one-by-one and test the results. When it fails you'll have found the culprit. Yes it's time consuming and it's the same process a developer will do to validate your environment. The more addons, the more time.
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