Update: I made a complete new setup on a newly installed target machine. The ansible-playbook passed without errors. Anyway, I get a blank page. There is o error in any log, except one warning on the target machine in /opt/openwisp2/wifi-login-pages/log/server.log:
yarn run v1.22.18
$ node config/setup.js && node config/build-translations.js
warning Cannot find a suitable global folder. Tried these: "/usr/local, /var/www/.yarn"
Could this be the reason? Thank you.