How to get correct grammar in the bibliography of a German document when the editor is an association?
If you are looking for a one-shot solution, you could add
execute={\expandafter\def\csname abx@lstr@byeditor\endcsname{Herausgegeben vom}}
to your bib-entry.
A more general solution would need some switches in the language strings (\ifwhatever{vom}{von}) and some bib field to set this switches -- possible but unclear if it is really worth the time ...-
This is one of the limitations of biblatex
's localisation system.
As a dirty fix you can include 'dem' into the editor field
editor = {{dem Fachverband Latexology} and Doe, John},
This will give good output only if the editor is used in the 'herausgegeben von ' position. If it is used as in ' (Hg.)' this will not work correctly.
Since the citations give the short form you can use
editor = {{\ifbibliography{dem }{}Fachverband Latexology} and Doe, John},
to fix that.