Sorry to interfere now, but I might be able to contribute something to the solution of the problem:
As I understand, the hotend ist constructed in such a way, that you screw the nozzle with its thread into the aluminium block from the bottom At the top of the hotend, two screws are used to attach a metal tube to the aluminum block, into which the filament is inserted by the stepper motor and in which it begins to melt.
In the “normal” or not-leaking state, a circular tongue at the end of the metal tube comes into direct contact with a circular groove in the nozzle when it is screwed in.This is how the seal is achieved.
If this seal between the the metal tube and the nozzle is not tight, there will be an extrusion of molten material at the top of the hotend, or from below, above the nozzle.
Have you screwed the nozzle tight enough into the hot aluminium block of the hot end? You might need to apply a lot of torque (Caution, support the aluminium block to not damage the hotend!)
If this does not help, try to disassemble the hot end into its individual parts (nozzle, aluminium block and metal pipe from above) and then put the parts in acetone for two days. Then try to scrape off the residue of the molten and burned plastic inside the hotend, and the end of the metal tube, that faces the nozzle and from the threaded end of the nozzle.
Maybe the metal pipe has been pushed too far up in your case. The metal sheet, that holds the metal pipe in the aluminium block would then be bent as in my case. Try to bend the metal sheet straight again, so that the metal pipe reaches a little bit further inside the aluminum block.
Hopefully, the groove comes back into contact with the tongue, and thus the connection is sealed again.
@Zortrax: Maybe this problem could be solved by a more rigid connection between metal tube and aluminium block, or another type of Seal between metal tube and nozzle.