Peugeot 107 Turn Signal Malfunction

Sounds like an electrical fault, which can be tricky. When you say 'cable or plug connection,' have you inspected the bulb holder itself? Sometimes they corrode or the contacts get bent. Also, have you tried replacing the bulb, even if it looks okay? A faulty bulb can sometimes throw weird errors.

I checked the bulb visually, it seemed fine. I did not replace it yet. Where exactly should I check for corrosion?

Corrosion can appear as a white or greenish deposit on the metal contacts within the bulb holder. It can also be present on the pins of the bulb itself. If the visual inspection didn't reveal anything and you're getting error messages, it might be worth taking it to a workshop with diagnostic equipment.

Thanks. Will do. Local shop confirmed the diagnosis and replaced the defective plug and cable for 95€.