The order function in data.table (or forder according to the error message) doesn't work as expected. Replacing it with base::order works though:

DT <- data.table(a = 1:4, b = 8:5)
for (i in c("a", "b"))
  print(DT[order(DT[, i, with = FALSE])])

This gives the following error in the current data.table development version (1.9.3):

Error in forder(DT, DT[, i, with = FALSE]) :
  Column '1' is type 'list' which is not supported for ordering currently.