Disney+ Watchgroup not available?

Hello,

I wanted to watch Titanic on Disney+ with a friend earlier.

We have created a watchgroup for this purpose, but if you click on the respective link, you get the following message:

" This GroupWatch is unavailable – The title you were invited to stream together or the GroupWatch feature is not available to you."

We both have our own accounts, and other titles work, so is it Titanic? Disney+ is only open in one window for both of us; we've tried Opera and Chrome. The creator of the GroupWatch could take a look; the message appears when you click on a link.

But why shouldn't this title be available for GroupWatch, since we both pay and can watch it individually?

Is there a list of titles that are not available for GroupWatch?

Kind regards, Isa