Not sure if this helps but...
Following a recent change of ISP I've just changed host for another of my sites with an embedded Nabble app. It meant I ended up with my app embedded in two totally different sites. It turns out that was when that I found Nabble offering two "Redirect to" options. I simply selected the option for the new version of the site and thereafter the old site disappeared from the "redirect to" option list.
Maybe you currently have a choice of two so you can re-select the new site?
Maybe you need to remove the redirect to allow Nabble to detect the site where it is now embedded, because it sounds as if Nabble is attempting to redirect it to a site that doesn't exist so just ends up releasing the redirect?
Otherwise, pass! This may be one for Nabble to re-set.
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Once the admin for GregHelp now deleted.