Just to add to the list: I saw the same error with "chrome/browser/process_singleton_posix" when updating to 125.0.5729.12 on Ubuntu. I "fixed" it based on @zibi1981 suggestion above, as follows:
opera --ozone-platform=x11From my crude understanding, it would again point to wayland vs x11 issue, but anyway, just sharing in case someone else ends up here after seeing opera crash on startup in Ubuntu.