IIRC, array rank reduction error is when certain nodes cannot accept an input where the levels are inconsistent, meaning there are elements in and out of nested list, at all @L’s instead of a consistent level. For example using a list like:
[1,2,3,[4,5,6],7]; where the elements are at different levels would give you an error but
[[1,2,3],[4,5,6,7]]; would not because it is consistently 2 levels deep.
The best way to resolve this is to use a consistent leveled input. If that is not possible, a workaround would be to flatten completely, do the changes you need to, then restructure the new list to match the old.