Hi @marc73, thanks for reaching out to us!
I would try adding the site directly from the Wordfence plugin rather than from the Central side to see if that changes anything. First, clear out the site if it appears in your Connection Issues tab.
Now head over to your site and log in as an admin. Navigate to Tools > Diagnostics > Other Tests > Clear all Wordfence Central connection data. Clear the connection data and then from the Wordfence Dashboard, click on “Connect this site” in the Wordfence Central widget.
https://www.wordfence.com/help/central/connect/#troubleshooting-connection-issues also has some troubleshooting steps you could follow.
Connectivity issues can occur if you’re hiding the /wp-admin page, or the REST API is disabled or blocked, but I think your issue sounds like a previous temporary connection issue could be the cause.
Thanks,
Peter.