Since you say that the MCU crashes even with out the load connected to the relay contacts, then most probably it is due to capacitive coupling / or leak via the PCB tracks . I do hope that basic guidelines for mixed power tracks ( Ac 230V , DC 3 V) have been followed while designing the PCB...

One simple test can tell if this is the problem. Pull the relay out of the PCB ( or use a different one ) ; keep it outside the PCB; now try driving it after connecting the AC power to the relay contacts with wires. In all probability it may not crash your system.

Raghu