Thanks for the info Jûrgen,
I added a 350cp mantle now and I will try the latern tonight...so that I can see if anything goes wrong...the nozzle is secure and all is clean for the next start
The halo with the 500cp mantle stayed there and got worse during the 10 minutes that I left the latern burning...and that's a bit too long I think..The mantle was well shaped after burning it down and no holes were detected...the pressure was also ok during the burn...