The pianist Siegfried Mauser was a major figure in the German music scene until his deep fall – a figure who was "incredibly well networked and played in Sloterdijk’s 70th just as well as he played in committees, juries and advisory boards in different countries", as "Die Zeit" wrote. Mauser headed the university from 2003 to 2014, after which he was rector of the Mozarteum University in Salzburg until 2016. It was the year in which Mauser was sentenced to one year and three months probation by the Munich District Court for sexual assault of a colleague. The appeal court later reduced the sentence to nine months.

In May 2018, the Munich Regional Court acquitted Mauser of the charge of raping a female job applicant, but sentenced him in three cases of sexual assault of an opera singer to two years and nine months imprisonment. The decision is not final because both the public prosecutor's office and the defence have appealed to the Federal Supreme Court. Mauser's attorneys consider the evidence to be insufficient, as testimony stands against testimony. In addition, the sentence is thought to be too high.

Mauser's planned appearance in Bonn did not take place in autumn 2018 after protests arose. "The Internationale Beethovenfest Bonn gGmbH reacted to the conviction," said spokesperson Barbara Dallheimer. "A performance at the Beethovenfest 2018 was cancelled by mutual agreement between Mrs. Wagner and Mr. Mauser".

But on November 22, Nike Wagner jumped publicly to the side of her musicologist friend. At an event of the Rhein-Neckar-Zeitung in Heidelberg's Theater, she declared, according to media reports, that Mauser was the victim of a witch hunt and a "malicious intrigue from inside the Musikhochschule". In addition, she said: "Women who want a job are not always just angels.“

The composer Moritz Eggert, a former colleague of Mauser's, then sent an outraged message to the artistic director at her private e-mail address. "You must know the other side," wrote Eggert, "before you repeat his protective assertions without having really verified their truthfulness.“ On December 17, Wagner forwarded the mail to Mauser from her official address and wrote: "Dear Sigi, as agreed. I did not reply to Eggert. Cordial, Nike". Sender: "Nike Wagner, artistic director".

Due to the mail, Mauser's lawyers obtained an injunction against the composer at the Munich Regional Court in February, who is not allowed to repeat several accusations from his mail for the time being. However, Eggert wants to overturn the rush decision in the main proceedings. "I am confident that I will be considered as correct on all counts," said Eggert, who is writing a youth opera for the Beethoven Jubilee Year 2020 on behalf of the Bonn Theatre and has composed for the Beethoven Festival before. But the lawyer's fees are annoying. "That Nike Wagner stands by her friend Mauser is fine," says Eggert. But the fact that she forwarded the mail from her office address violated the neutrality and integrity that must be demanded of an artistic director. He threatens with a criminal complaint against Wagner, the receipt of which, however, the Bonn public prosecutor's office has not yet been able to confirm. Wagner's lawyer told the GA on Wednesday evening that the forwarding of the mail was in no way a criminal offence.

Nike Wagner herself does not comment on Eggert's mail. She also left the GA question open as to whether she would stand by her statements in Heidelberg and instead published a press release. Mayor Ashok Sridharan, who sits on the supervisory board of the non-profit International Beethovenfeste gGmbH on behalf of the city, commented on the mail affair: "Professor Wagner had called me herself and informed me about the matter. We have agreed that she will use a private account in future. I consider that to be implemented. By the way, the statement by Mrs. Wagner in the Mauser case says it all for me.“

Deutsche Welle, as another shareholder of the Beethoven Festival, did not comment "on statements made privately by Mrs. Wagner". Nike Wagner's contract as artistic director runs until next year. The city of Bonn does not reveal whether an extension is already being negotiated.

Mauser's successor as president of the university in Munich reacted to Wagner's statements in Heidelberg with an open letter. Bernd Redmann criticized that such attempts at relativization "discredit all those affected by sexual harassment and violence".