RCBO makes one lamp blink
Yesterday I finally got pissed enough by the fact that I don't have a working light in the dining room that I got myself to go after this. I'm happy to say that a) I sorted it out b) I think I even understand the situation to some degree.

As you can see on the diagram, this problematic light was toggled by 2 3-way switches, let's call them switch A and switch B. We also have circuit 1 (green) and circuit 3 (blue).

From what I understand, the problem arose from the fact that switch B is connected to the live of circuit 1 while the light is connected to the neutral of circuit 3.

On top of that, the live of switch B is also shared with an outlet, also in circuit 1. Needless to say, this outlet is also wired to circuit 1's neutral. To be honest, I'm surprised all this mess actually worked until I installed the RCBOs.

So anyway, there's no easy way to fix this unless you're willing to do some rewiring and chiseling. That I most definitely wasn't willing to do, so instead I ended up eliminating switch B and converting switch A to a single pole switch. Now the light is wired to the neutral of circuit 3, and also to the live of circuit 3 via switch A. The aforementioned outlet is now connected directly to circuit 1's live, without a switch in between.

Everything seems to work perfectly. I checked the wires with a phase tester to see which one has phase and when, each RCBO powers the corresponding outlets and lights, and nothing else. Then I also checked with the "RCD tester" (shown above in OP) if the main RCCB and the circuit RCBOs are doing their work - the tester always trips the corresponding RCBO and the main RCCB, too (but not other circuits' RCBOs). So things seem to be fine now. Thanks everyone for the help!