Hi Yannick,
I hope you have/had a great holiday - depending on when you see this. If you don't see this until after, I may already have it solved by then.
The 4SEO pop-up box opening full screen and blank over the pages on this sandbox site. I'm not sure what the issue is - or at least how to troubleshoot it.
https://xxxx.fcp.co/
user: xxxx
pass: xxxx