Actually, the button doesn't seem to work for me, either. The %1 doesn't get turned into the selected filename for some reason. I'll ask Jon why that is.
Because %1 has significance internal to batch files, in the Call :Label syntax. This is as a result of an Issue Report I submitted. Use {File$} or {Filepath$} instead.
%1 does still work in other types of command but, in Opus 9, %1 no longer turns into the filename in MS-DOS batch commands because it causes a conflict with the meaning of %1 in batch files, as Ken says.
This is fine as the only place you need to use %1, instead of {filepath} etc., is when you want commands which will work in both Opus and Explorer. Since MS-DOS batch commands don't work in Explorer anyway nothing is lost by the change, but some old buttons like this one may need slight updates.
Edit: I've updated the root post with new buttons that work in Opus 6, 8 and 9 and deleted most of the intermediate posts (which are no longer relevant) to tidy things up.
Just a related note, %L also will no longer work in MS-DOS Batch Functions, for the same reason. But Like Nudel says, you lose nothing hereāit's really only useful for Functions that need to work in Explorer and Opus.