There's nothing special about the Nazi Party wrt copyright law
So the analysis follows the same route as it does for anyone else.
Under German law, copyright is inalienable - there is no corporate ownership of copyright in Germany, the work always belongs to the author. Other people, including corporations, can be licenced to commercially exploit the copyright but they cannot own it.
The Nazi Party never owned the copyright, however, there is an implicit term in employment contracts that an employer (and it is unclear if Schweitzer was an employee or a contractor) has an exclusive commercial licence in works created by their employees subject to the things that can't be transferred (e.g. the author's right to be identified). This term can be made implicit or waived. Of course, for a contractor, the licence needs to be dealt with in the contract.
However, licences granted before the mid-1990s could not grant rights in media that did not exist: whatever' the Nazi Party's licence, it cannot have covered publication on the internet. Notwithstanding, the end of the Nazi Party meant the end of any contracts or licences (which are different things in civil law, unlike common law where a licence is a type of contract) they were a party to.
Schweitzer died in 1980 so the work is under copyright until 31 December 2050 inclusive. It is owned by his heirs, whoever they are.