While I was looking for a source about Kronecker's Jugendtraum, I found this document: Auszug aus einem Briefe von L. Kronecker an R. Dedekind vom 15. März 1880.
The bottom of the given page has a note: Jacobi, Werke, Bd. 1, S. 266. I found this, but it contains apparently no information about this letter from Kronecker to Dedekind. Does anyone know how this reference to Jacobi's works should be understood?
Moreover, we have only here an "Auszug" (= excerpt) of the letter. Is the full version available anywhere?