I changed the permissions and checked, as I said before. I even tried the alternative configuration from the NodeBB scaling guide. That didn't make any difference. There are missing directories and I don't know why.
I'll delete NodeBB and move on. Thanks for trying to help guys.
@MagicVish No, no. For the field Which database to use do not enter redis but enter 0. You already chose redis earlier in the setup. This field instructs NodeBB which redis database to populate and use. The databases start at 0 and increase incrementally. If this is your first and only redis database 0 is the answer.