Protection Circuit goes into over current mode

Hey,
Im currently working on my Mag 3D Mod, which includes a 3S 18650 pack from old laptop packs.
The problem I have is, that if I switch on the flashlight with the High Setting ( normally around 1-1.5A ) the voltage drops to about 1V.
The Strobe mode blinks 1 time, then again 1V. The low mode ( around 0.2A ) works fine!
The confusing thing is, that if I short it with an 4 Ohm Resistor ( ~ 3A ) it works fine, only drops to 11V or so.
I tried to add a 100uF Cap but no change here…
Any Ideas?

The protection board im using is the Keeppower XZD-3S1550 ( Datasheet: https://www.akkuteile.de/tpl/download/3S-1550.pdf ) which should only go into over current mode if its loaded with 26A..

Well, let's ponder this. We see the pack working fine with the same switch, so it probably isn't a switch short.
With a resistor it supplies 3A without problem. Have you measured this current just to be sure?

Draining an 18650 down to 0.3v per cell is startingly high current for decent cells. However, an instant drop from fresh (4.2v) cells to 11v at only 3A is quite a bit. That has your cells already near nominal voltage. Are they healthy? This circuit is probably tripping on a low-voltage problem rather than a high-current one.

As an aside, that may not be an official datasheet. All our contracts and documentation from Chinese companies use the company chop on the original of a document. However, I don't get such clean PDFs, just scanned copies of real pages that someone stamped.

If you still have an incandescent bulb, or perhaps even an LED, the inrush current is probably what triggers the protection. Inrush in an incan can be 3-5 times normal current.

So, I hooked up my scope on the battery side, on the low mode it hat a sawtooth form with an Amplitude of 1.5V, offset not measured..
Ive got an LED hooked up, so the inrush current shouldnt be that high... If I switch to high or strobe mode I can see exactly the same thing on my scope..
Tested 2 diffrent batterypacks, one with old LG cells and one with old samsung ones. They are working perfectly fine, as far as I can see.

One possibility I've not seen mentioned yet is that the protection circuit on the cells might have gone bad and it's tripping on the inrush, your resistor partially resolving this issue hints at this being a factor to some degree. You might wish to try new 18650 cells first to eliminate that possibility.