Hi Miguel ! Good news, it works again !
After set the memory to 4GB I restarted the indexer, then I took a look at the journalctl -xe | grep indexer and I saw lines that states that the indexer have some problems of rights for execution, so I did something you are not supposed to do ( but I did it because it's a test server) : chmod +xwr /etc/wazuh-indexer/* and chmod +xwr /lib/systemd/system/wazuh-*
I think that my problem was a mix between the memory and the rights (because there were no such lines about executions rights before).
Thanks a lot for your help !!!
Have a beautiful day.
Duncan