The 30 second message can be turned off (see below), but it is a sign something went wrong with an OS install or partition restore or something along those lines.
Was it already there before you tried to reinstall the OS, or was it a result of trying to reinstall the OS?
If trying to reinstall the OS caused the 30 second message, then I would suggest trying again, but this time making sure to not use adb and fastboot from your Linux distribution like the guide suggests, but from the current Android SDK Platform Tools.
(Or use a Windows computer. I don’t know whether it’s still the case right now, but there were bad fastboot versions around in Linux a while ago, causing trouble.)
If the OS installs successfully and still reboots randomly, it’s a support case … https://support.fairphone.com/hc/en-us.
Or you could return the phone within the cool-off period.
… works once Android or TWRP are booted.