this sounds like an extremly overheated hotend. With a lot of liquid filament you can flood your guiding tubes, generating a blockage of the filament anywhere.
A side effect if this overheating is the risk of a clogged nozzle, which appears when the filament will burn in the nozzle.
It’s a pity that you got the load of problems with the K8400 after your first print. To go on further you need to dismount the heating block completely. You may try to remove the filament from the PTFE tubes mechanically (2mm drill). Depending how much you damage it by this procedure you may re-use them. To check the nozzle you may search some topics in this forum. If you’re shure that the nozzle is damaged you may try a ticket to Vellemann to provide a new one since it sounds like a misfunction of the printer.
But when you re-start with printing don’t believe any temperature settings given elsewhere. The weak point of the K8400 is the large variation of the thermal coupling from the heater block to the thermistor. Depending on the tolerances of your parts there can be a temperature error up to 10%, which means, the hotend is 10% hoter than the temperature reading. When switching on the fan this error can increase: the air flow is effectively cooling down the thermistor, so the heater block can become hotter.
The next uncertainty is the extrusion temperature for your filament. Most of the suppliers are giving a temperature range, well knowing, that there are no real calibrated temperatures in the 3D printers. It can be taken as a first hint where to settle the actual temperature.
So when you re-start search the lowest temperature, where some extrusion is still capable. Then increase the temperature (e.g. in 5°C steps) until the filament is flowing continuously (when it doesn’t stop flowing after extrusion end). I personally recommend the last temperature step before this continuous flow as the preferred printing temperature. Please have in mind, that this temperature will vary even for one type of filament, e.g. PLA. Different vendors and colours will behave slightly different, so you should run this procedure for each new material.