You can submit issues in the issue tracker of the repository.
crontab-ui is running but is not accessible on browser - This is usually because the place where your crontab-ui is installed does not give access to others. It can be resolved by either giving permission to the user (Recommended) or running crontab-ui as root. Refer this
Hosting crontab-ui : it works on localhost but not outside the server - You have to host it using nginx, apache2, etc. Refer this.
crontab-ui stopped working - It can happen that your crontab-ui can stop working for some reason like adding incorrect jobs or timings. You can try resetting crontab-ui by running crontab-ui --reset.
Where is my root node_modules folder - You can find it by npm root -g
Mailing related issue - Refer this.
Run crontab-ui as a daemon - Install pm2 using npm install -g pm2. Then just run pm2 start crontab-ui