This is a sign that the DSS backend process is restarting. The most probable cause is an out of memory situation. You can check the "run/backend.log" file. Look for the "DSS startup" messages, and go up, until you see the cause for crash.
You might need to increase "backend.xmx" (default 2g) and "backend.permgen" (default 200m) as detailed here: https://doc.dataiku.com/dss/latest/installation/java_env.html