OK, so after 4+ years ( 🙂 ), I donated the printer to a friend and came back to this issue. I will leave the fix here, if others hit it .
Disconnecting the thermistors had no effect. The indicated temp was still around 66 degrees now (yes, it degraded over time).
Compared bed, E0 and E1. Measured resistors, voltages. Short circuiting indicated 2000C, as expected. E1 was different from bed and E0 in measured values (3.3 V compared to 3.1V on the others the measuring pin).
Long story short, replaced C19 (the bad guy, or C18 looking at the picture. The one that filters the E0 sensor signal) with C65 (donor) and everything works as in the first day.
WhatsApp Image 2024-10-22 at 1.15.23 PM.jpeg