Comment # 8 on bug 1178596 from Fabian Vogt

(In reply to Fabian Vogt from comment #5)
> Since
> https://invent.kde.org/plasma/plasma-desktop/commit/
> d1f62c16c878a665c6e548f7a99b6fab21882e6d, EmailClient can refer to a
> .desktop file instead.
> 
> xdg-email should do this in "open_kde":
> 
> if [ "$client" != "${client%.desktop}" ]; then
>     client=`desktop_file_to_binary "$client"`
> fi

That's actually not the whole story - that code path is only used when it
contains "thunderbird". It seems like the fallback path using kde-open5 is
affected and simply kde-open5 mailto: also fails with that error. So this is
most likely a bug in KIO or the scheme handler is not set correctly anymore.