Sddm stopped working after Update

After I’ve updated my NixOS System again (I’m on unstable branch), I didn’t saw directly the login manager, so sddm stopped working and I don’t know why, in journalctl I couldn’t find any Error. If I tried to start sddm in the console with the command sddm, I’ve got this Error: Failed to read display number from pipe. I couldn’t find any really helpful answers. The Only thing that I’ve found was a Solution in the Arch Wiki, where it said that there were and old Xorg Configuration and after deleting it, sddm startet again. But I didn’t found any Solution for me. And I don’t know how to Troubleshoot that. Can somebody help here?