I'm rooting for you @walbin, really wish you could submit that one to NS directly to see if I'm right.....

Hi, I am able to perform update line item from NS but having the same issue while trying to update from SuiteTalk api. For now I am removing the existing line from sandbox and reinserting the lines again in sandbox which resolved my issue, but it is a kind of a patch for me, which is not good approach as per my point of view.