Thanks for the tip! Now I can see all the things happening ^^ :D

According to selfoss logs last error was yesterday at 17:30 due to mbstring problem. So no further information about the feeds or the updates.

However, early on I rebooted the Pi and now is fetching all the feeds so I guess there was some bit not fully loaded. Also, the cron schedule does work now every 30 minutes as I originally wanted.

Unfortunately, not everything are good news xD The error "Can not update sources: Not sources found." has come back when trying to do an update manually.