We have been unable to recreate this issue and are stumped as to where the problem may be. We have tested on multiple Joomla 3 and Joomla 4 sites and the modal works on all. However, every site has different templates and components that can result in conflicts, so we simply need more information.
Think Ad - Thank you for sharing the Javascript the helped make the modal work on your site. On further investigation, if we switch the template to protostar on your site, the modal *does* work. You can try yourself by simply going to your website and adding ?template=protostar to the end of the URL. The page will look different, but the Connexion button does pop up properly.
Most importantly, when loading the protostar template, the templates/bd19/js/template.js file is *not* loaded. That template.js file is where your developers added the fix to make the popup work. That indicates there's something else on the site that's causing the conflict. Unfortunately, we can't see the issue with the 'fix' code on the page, so we can't narrow down the cause.
Anyone else with issues - For us to investigate further, we really need to see more sites with the problem. Please post (or use the Contact Us link at the top of the page) a link to a page on your site with the issue so we can dig in deeper.
Thanks,
Alex