Even though the PCB has conformal coating, it somehow has corrosion formed on it. I have seen this fault at two different irons. I think it is a very common issue.

Use 99% isopropyl alcohol and a soft brush to clean the oxidation. In my case I had to use brake cleaner, too. I am not sure if this has damaged the existing conformal coating.

If you have access to paint on conformal coating, applying some may be a good idea. I haven't done it.

Use a hairdryer to dry the PCB before re-assembly.