Ok, the following converts the data to a Vector{Matrix{Float64}} of size (12000,):
d = [hcat(c[:]) for c in eachrow(data)]
d[:,1] ~ Normal(predicted[:,1])
d[:,2] ~ Normal(predicted[:,2])
but this was not the solution after all - I still get the same BoundsError in the same place.
Any suggestions of what else may be wrong with my code/approach would be very welcome.