Flash hiders are not listed in §17 WaffG nor in the KMV, and not mentioned in the WaffG-DfV.
They might be in the criteria for determining the status of semi-automatic rifles, but were not mentioned in the recent Anfragebeantwortung, so I doubt they're there.
Near as I can tell, there's no legal basis for them being illegal.
They are not illegal per se, but, together with bipods, bajonet mount, folding stock, on the new 5.56 semi-auto carbines OA15A , Sig 550 and AUG-Z they indicate "war material" (according to the "Erlaß des BMI vom 25.März 1980").
The Vienna gun dealer Seidler, who was deeply involved in the legalization of these carbines, refuses to mount flashhiders on them because of that.
If you have another source or plan to ask the BMI please go ahead.