Happened to me too: i was on a previous arc loader (non "next" version) and in the wiki says it's required to select "force reinstall" DSM at boot when upgrading to this new "next" version.
A force reinstall keeps your data and your configuration, so no need to worry.
So after a few failed attempts to reinstall DSM (system partition became full after uploading multiple times the .pat because the wizard complained about the file being damaged), my box has the control panel working again