I don't know why you have triple and earth I can only guess that both switched line and permanent line are taken to the second lamp. If I assume this then looking at picture of your ceiling rose you have.
Switched line (two connections end) Black, Yellow, and Brown. Permanent line (centre) Four reds. Neutral (three connections end) Two blacks and two blues
So the slave lamp should have going from that. Switched line (two connections end) Yellow, Brown. Permanent line (centre) Red Neutral (three connections end) Two blues.
You may also have a feed to another lamp other wise can't see why triple and earth is used.
Since you say other lamps acting odd I will guess you have got the blacks mixed up. Two of the blacks are neutral and one is switched line. With power off and an assistant with a multi-meter or some other set up could even be a door bell you need to "Bell out" the cables to find out which one goes to the switch.
I say assistant as want some one to flick the switch on/off while you see if continuity between red and black also switches on and off.
I did once write out a test sequence to work it out without belling out but I was banned from this site for my trouble. Some one claimed it could be dangerous. Then that poster disappeared and failed to help anyone.
What I would warn against is using trial and error as in one of the three combinations it would cause a direct short and likely damage the switch and also blow fuse or trip MCB. In theory the black switched line wire should have a red sleeve on it. Or at least some red tape.
The reason you have lights flashing is they are now wired in series rather than parallel due to blacks being incorrectly fitted.
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