Sony Ericsson never initially told people that unlocking the bootloader of their new Xperia smartphones was a one-way process. This meant the DRM keys were wiped and you weren’t able to update your firmware via SEUS.
Well Blagus from xda-developers discovered a way to re-lock the bootloader for his Xperia arc. You won’t be able to get your DRM keys back, but you can update firmware from SEUS again. There is a handy tutorial on how to do this over at the Android Soul.