you probably know more than me, but i’d suspect contamination in the cold side, thus

• deep clean everyth ing and make new starsan solution (or something similar)
• check speed of cooling after boiling. final temp should be ok, since it’s apparently cold enough to not kill t he yeast.
• check ph of wort (somwhere i’ve heard that people adjust ph using lactic acid or citric acid, but i’ve never done that in brewing beer.)

good luck!