Hi Niko H.,
Unless you're having intermittent internet issues, something seems to be preventing WebSocket communications from reaching the client. If that's the case, you'll probably also see users being always marked offline or you might not receive new messages posted. Here's a link to some possible solutions to the problem: http://docs.mattermost.com/install/troubleshooting.html#please-check-connection-mattermost-unreachable-if-issue-persists-ask-administrator-to-check-websocket-port