Is R1 really 1 ohm? Then what purpose does it fill?

Yes, it is 1 ohm. Before I reverse engineered the circuit I thought it is current sense. But it is not. I changed it to 0.5 ohm and current did not change. Then I reverse engineered schematic. I assume it is there to limit the current in case that driver failed.

Could it be a damping resistor, to reduce ringing on the output of the driver?

Have you think about making a stupid current source by hand: jfet + resistor, or npn + resistor + zener ?

Hmm, I don't think failing parts is necessarily the problem given the pics. The solder job is truly awful and besides you got corrosion of some kind all over, to the point where I would suspect that the board is water damaged.

@Lundin There is no failing part. It is working correctly. My plan was to increase output power (so increase current). But there is a device that outputs constant current, which I would like to replace for part with higher constant current. Thus I would like to find such part (what is name of it generally), because I was not able to find no similar part. Yes, some soldering is ugly there, I was soldering there different value of R1, which made no difference in current.

It looks like a board where anything can fail at any moment though... Anyway, why not just pick a brighter LED? So much easier.

@Lundin Well, my question was not focused on alternative schematics or options how to modify that circuit. My question was really specifically about that one device, because I can't find any such device. I could use it in different circuits as well, it could be handy. btw. I already changed LED for more powerful one, that's why I'm focused on that constant current device now.

@Chupacabras The 90mA current is an odd case, most LEDs are rated at 20mA. Also, you can get extreme super bright LEDs that shine like the sun with several candela at 20mA, why focusing on the LED is easier.