I've seen it on a machine on which I did an upgrade, but also on a new machine, with the following:
Steps to reproduce: upgrade to the indicated versions.
I can see a "403 Forbidden" error visiting the NextCloud button Application within NS7. As I said, it can be seen here and here; the former was upgraded from previous version, the latter was installed from scratch. Other websites on the same machine still works (example1 and example2).
A listing on the main directory shows nothing helpful:
# pwd; ls
apps config core data lib
Can anyone confirm?