I finally found the error myself :
A = [1 2 3 4 5] ;
string(A)
ans = 1×5 string array
"1" "2" "3" "4" "5"
num2str(A)
ans = '1 2 3 4 5'
whatItNeeds = strcat('[',num2str(A),']')
whatItNeeds = '[1 2 3 4 5]'
The latter variable is what setBlockParameter needs as input to eval it into a matrix. string( ) , that I used in my example, transforms each elements of the array into a str. num2str( ) ransforms the whole array into a single str character, but misses the '[' brackets.
Hope it helps someone else !