It used to work before on my old computer..

I don't see anything special in the docs https://develop.spacemacs.org/layers/+filetree/neotree/README.html

As treemacs has been the default in develop for many years, perhaps there is a bug with the neotree layer.

I commented out treemacs layer and added neotree and restarted Emacs. Neotree seems to work okay. Using neotree seems to limit some of the LSP mode features, which use treemacs. From the *messages*buffer:

(Spacemacs) --> refreshing package archive: melpa... [1/3]
Contacting host: melpa.org:443
Importing package-keyring.gpg...done
Contacting host: melpa.org:443 [2 times]
Package refresh done
(Spacemacs) --> refreshing package archive: gnu... [2/3]
Contacting host: elpa.gnu.org:443
Importing package-keyring.gpg...done
Contacting host: elpa.gnu.org:443
Package refresh done
(Spacemacs) --> refreshing package archive: nongnu... [3/3]
Importing package-keyring.gpg...done
Contacting host: elpa.nongnu.org:443
Package refresh done
(Spacemacs) --> installing package: neotree@neotree... [1/1]
Parsing tar file...done
Extracting...done
  INFO     Scraping files for loaddefs...done
  GEN      neotree-autoloads.el
Checking /Users/johnny/.config/emacs/elpa/29.2/develop/neotree-20230815.219... [3 times]
Compiling /Users/johnny/.config/emacs/elpa/29.2/develop/neotree-20230815.219/neotree.el...done
Wrote /Users/johnny/.config/emacs/elpa/29.2/develop/neotree-20230815.219/neotree.elc
Checking /Users/johnny/.config/emacs/elpa/29.2/develop/neotree-20230815.219...
Checking /Users/johnny/.config/emacs/elpa/29.2/develop/neotree-20230815.219/icons...
Done (Total of 1 file compiled, 2 skipped)
Package 'neotree' installed.
(Spacemacs) --> deleting treemacs-projectile... [1/9]
Package 'treemacs-projectile-20230703.1929' deleted.
(Spacemacs) --> deleting treemacs-persp... [2/9]
Package 'treemacs-persp-20230703.1929' deleted.
(Spacemacs) --> deleting treemacs-magit... [3/9]
Package 'treemacs-magit-20230830.1936' deleted.
(Spacemacs) --> deleting treemacs-icons-dired... [4/9]
Package 'treemacs-icons-dired-20230930.1141' deleted.
(Spacemacs) --> deleting treemacs-evil... [5/9]
Package 'treemacs-evil-20230822.2039' deleted.
(Spacemacs) --> deleting treemacs... [6/9]
Package 'treemacs-20240126.642' deleted.
(Spacemacs) --> deleting pfuture... [7/9]
Package 'pfuture-20220913.1401' deleted.
(Spacemacs) --> deleting lsp-treemacs... [8/9]
Package 'lsp-treemacs-20230811.611' deleted.
(Spacemacs) --> deleting cfrs... [9/9]
Package 'cfrs-20220129.1149' deleted.

Example of neotree working.

I gave up for now and trying to use treemacs…