I spend already a huge amount of time setting up Gitlab on a Synology with the pakage provided by Synology.
The problem was in the setting of the SMTP server. Nothing worked, and the confirmations mails were not send.
Finally, I realised that I could make directly the confirmations from the root account.
Now it is working but the SMTP is still not properly set up.
Is there a way to configure the SMTP through the GitLab GUI ?