Hmm, I use effektiv-spenden.org & that’s the only option I know of. Maybe you could try emailing them to ask about EA Funds? The Effective Altruism Foundation, which used to offer this service, were always responsive & helpful when I contacted them.
If I remember correctly, German law requires that the charity that writes the donation certificate has control of or at least documents how the donations were invested. (Not sure which of the two requirements it was.) I think that is why the forwarding to the EA Funds had to be discontinued. The EA Funds pool all the donations and then allocate them at their discretion, so it’s not possible for the German charity to control how the donations are used, and it’s probably difficult or impossible to know what project was supported to what degree by which donation.
Yeah, what Denis wrote sounds correct to me.