Version 1.5.3 was a mandatory update to PMS. At that point, how PMS communicates with Plex.tv was changed. It was no longer possible to support both old and new communication protocols.
Thank you for the quick answer.
I tried to upgrade ram (to 8Go) but still the same problem.
Impossible to start any package from 1.5.3, I always have the same message “Failed to launch package repair”
And it’s impossible to downgrade DSM to 6.1-15047.