Hello nzapponi,
I reached out to someone who said there might be something wrong with your octocoupler/reference circuit. They said the 5V output and 2.5V reference leave little headroom for biasing the opto diode and TL432. The TL432 needs 1.0mA minimum and the opto-diode will need over 1.0mA. To fix this, they suggested lowering R21 to be much lower than 2.5k, around 1.2k.
Or the better solution would to be replace the TL432 with a TL431 or ZL431 with a 1.24 reference. Compensation will need to be adjusted also.
Thank you,
Vinh Tran