Thank you for your reply but no luck, same reboots.
I also tried to pull off the network cable (as explained on page 101 of the user manual) : no luck.
I also tried to remove the low-toner cyan cartridge in order to get an error code : no luck. Reboots seem to occur before checking the cartridges.
Maybe try powering it off from both power switches and back on to see if that helps.
After a month off, my Phaser 7800DN refuses to restart. Sometimes it reports that I have to turn off the power, wait 10 seconds and turn the power back on (rare), sometimes it goes into an endless auto restart/shutdown cycle (very often).
My only clues are that the cyan toner is very low and that I don't have full internet access right now (fiber access cut off, 4G fallback but not all ports available).
In any case, no error message and web interface obviously not available at this level.
Thank you for any resolution leads before calling a technician (I don't have a maintenance contract).