After donwloading "Redirection" Plugin, I did create a new german language file, did a rescan an opened the editor.
Than I did switch to plurals, entered both phrases and pressed the save button. All work fine no warning has been issued.
So make it short, i can't reproduce the error you described above. I have no idea, why this happens at your system. This warning will only be issued, if you press generate mo-file in case the entiered translation file is empty. But also generation of mo-file with only this plural editing works as expected at my machines.