Hi Zoojar,

You said "But then there is a diode soldered right on the relay !!!".

I believe that what on the relay is a kind of fast-decay diode or 1N400x, which is different from the Zener one. And don't forget, for the same purpose, there is also such a diode within ULN2003. So, try a Zener diode as I mentioned before.

If nothing works, probably you should try a different brand of MCU. Some MCUs are not so sensitive to be reset.

Regards,
Oliver