Hello @seppobastian 
That is really strange… without any information I would think that there is not enough RAM and no SWAP. So the the system crashes after a while…
However, have look at this first:
Then a log is useful:
journalctl --boot -1 --no-hostname | curl -F'file=@-' https://0x0.st
(after | is the command to upload the log)
to get the log from the local installation you have to chroot with this helper script from the live session:
manjaro-chroot -a