by Varenne Riocha.
Update: I had to change the site URL of both sites for unrelated reasons. Now I see that the first site appears in a frame inside the second site as the External tool activity. Not exactly what I wanted, I thought I exported a PDF, still.
I'm sure, I haven't changed anything related to LTI, perhaps it is the login - am 'admin' in both. I saw in the documentation that the user has to be the same. What does that mean? The same username? I have only manual registrations on both sites so far.