Ubuntu server become unreachable on LAN only

Am running an Ubuntu 18.04.1 LTS server. I can login to it either from my LAN or from the cloud with my router's port forwarding.

After a while of being up and running the server becomes unreachable via the LAN. I cant log into via Putty nor FTP. And that's from multiple devices I use to login to that server.

However, when this error occurs I can login from the outside, the cloud.

Logs from Putty have no info other than "can't connect." When this situation occurs I can't even ping the server on it's local IP address.

Rebooting it solves it all, but I have no idea what's causing this. Seems weird.

Thanks