Hi Florian,
legally, the German association "Freies Office Deutschland e.V." is the
current trademark holder, as TDF doesn't exist as legal entity yet.
Ideally then, reference should be made to the FOD Verein and not to TDF.
Not mentioning the name of the rights holder at all opens the person who
publicises such a mark on their product to allegations of fraud,
trademark infringement, and false assertion of rights (jurisdiction
dependent as always of course), including via third parties (i.e. it
doesn't require TDF to take positive action). Trademark law can be a
real pain at times :-)). In general, this is relatively low risk stuff.
Alex