This is being caused by NPM package socket.io in version 2.4.0.
It can be solved by downgrading socket.io to version 2.3.0:
- login to openhab
- become root: sudo bash
- change direcotry to: cd /usr/lib/node_modules/frontail
- downgrade the socket.io package: npm install socket.io@2.3.0
- restart frontail: systemctl restart frontail
As long as this problem exist in socket.io 2.4.0 and frontail depends on 2.4.0 this will pop up again in case frontail is being reinstalled