Failing to understand and to fix a PWM spindle controller circuit
Well you have what looks like 48 V DC being produced by rectifying the AC36V input supply. This connects to your spindle motor(?) and it might take a big current at start up thus several tens of volts may be forced to the top of R5 (100 ohms). R5 feeds an opto diode that is grounded and so you might see 20 or 30 volts for several seconds across R5. This will dissipate a power of 4 watts to 9 watts for some short period of time while the spindle device is running up to speed.

If you are using a surface mount resistor, it will likely melt. It seems to me that 100 ohms is too low as it will also force up to 300 mA through the EL817 opto and this is rated for 60 mA absolute maximum.

These numbers I have used might be out by a factor of two or three but I definitely smell a problem with R5 being underpowered and too low in value. 1 kohm might be more appropriate.


  Also, I do not need the STOP signal at the bottom (any clue what is it
  for btw? PWM feedback?)


No, because I have no idea what you have reverse engineered.