Hello @danbjoseph, thank you for the return.
In truth, I think the problem comes from a disk memory issue on GCP. I tried to reboot as it was explained there : 502 Bad Gateway nginx/1.14.0 (Ubuntu) while using ODK aggregate - #2 by yanokwa
But it's not working. The command line displays me : "Failed to write reboot parameter file: No such file or directory" It's the same with other similar commands like "sudo reboot -f". In my VM instance, there are serial port problems dealing with snapd.service or Snap Daemon that I don't understand and cannot fix.
If someone has an experience of GCP VM instance and/or a means to keep the same URL and host server with a new instance that would be great !