Why is MCB tripping after replacing Honeywell controller with Tado receiver

[sh0123]

Tried replacing Honeywell controller (ST6400C) with Tado wireless v3+ receiver. After switching the power of on, the Tado controller lights up and connects to WiFi. However, when the hot water or heating is activated via Tado app, the fuse in the central heating circuit blows off and the MCB gets tripped. (It's a conventional boiler - Glowworm Micron with hot water cylinder)

Appreciate if some experts can advise on the potential issue. Attaching before and after wiring photos. Thanks in advance .

 

[UNG]

Difficult to see what you have done, looks like you have flex feeding the new unit and the normal fixed wiring on the old backplate so the proble is obviously wherever you have joined the two

 

[sh0123]

Difficult to see what you have done, looks like you have flex feeding the new unit and the normal fixed wiring on the old backplate so the proble is obviously wherever you have joined the two

Essentially behind the backplate are Spliceline connectors wherein the fixed wiring are connected to flex (L, N, HW NO and CH NO) respectively. Tomorrow I'll try taking few photos and upload here.

The reason to use flex was CH NO connection was a bit tight to work given shorter fix. If you think that fix to flex might be an issue causing the trip, then I'll try connecting fix wiring directly to Tado receiver. Would that connections be causing the issue here?

 

[UNG]

Essentially behind the backplate are Spliceline connectors wherein the fixed wiring are connected to flex (L, N, HW NO and CH NO) respectively. Tomorrow I'll try taking few photos and upload here.

The reason to use flex was CH NO connection was a bit tight to work given shorter fix. If you think that fix to flex might be an issue causing the trip, then I'll try connecting fix wiring directly to Tado receiver. Would that connections be causing the issue here?

Fixing screws grazing cables and damaging insulation when being tightened are notorious for causing hidden faults in back boxes like you have everything is ok until the controller operates and you get problems