Shellly 1L doesn't turn off the light completely (or stays on with a dim light)

Hello everyone,

I hope you can help me with my problem,

I followed the diagram from Dimitar (on the Shelly Support Group on Facebook -- also attached) for installing a Shelly 1L on a 2-way switch (two switches controlling the same light bulb).

And the problem that I have is that, although the light turn off and off with both the Shelly and the switches, in certain combinations it does not turn off completely. It remains really dim, about 10% of the energy.

I've found someone with the same issue in the Shelly Support Forum EU (in German): Shelly 1L schaltet nicht ganz aus.

This is before:

This is after:

And this is the diagram from the Facebook grouped that I followed:

What could be the issue? How can I solve it?

Best Regards,