It would seem the fire actually broke out from part of Fjord Rafting rather than the backstage areas as had been previously assumed.
EP's French twitter wrote:
The criminal police investigations carried out following the fire at the leisure park at Rust and the preliminary evaluation of the results of the technical expert reports indicate that the fire seems to be in the "Fjord-Rafting" area.
The origin of the fire is probably due to a technical failure based on the current state of the investigation. For now, there is nothing to indicate theinvolvement of a third party in the incident.
The technical investigations of the fire experts will nevertheless continue for some time and the final conclusions are not yet established.
On CoasterFriends.de someone has drawn onto Google Maps the location of the tunnel (red oval) under the buildings complex, and the red circle marks where the flames were in the YouTube video above.
thats my demonstration, ive made for the german forum.