Hoyzer, a member of the Bundesliga's Hertha BSC Berlin, was registered by the German Football Association (DFB) as a referee in 2001, initially only officiating matches in the fourth-division Regionalliga Nord, where he made a total of nine appearances in the 2001–02 season. At the beginning of the 2002–2003 season, he was added to the list of assistant referees for the 2nd Bundesliga and made 13 appearances in the league throughout the season. At the beginning of 2003–04 season, he was added to the list of first officials for both the 2nd Bundesliga and German Cup (DFB-Pokal), and also continued to officiate matches in the Regionalliga Nord. He officiated a total of 26 matches in these three competitions over the following two seasons. He never officiated matches in the 1st Bundesliga.
Hoyzer's career as a referee came to an abrupt end in January 2005 after he was suspected of betting on a first-round German Cup tie between regional league side Paderborn and 1st Bundesliga club Hamburger SV on 21 August 2004. In the match, HSV took a 2–0 lead, but Hoyzer sent off HSV striker Emile Mpenza in the first half, and also awarded Paderborn two questionable penalties. Paderborn went on to win 4–2.
Hoyzer resigned as a referee and resigned his membership with Hertha BSC Berlin. The DFB soon announced that it was investigating whether or not Hoyzer fixed other matches in the regional leagues and second division. The German prosecutor's office in Braunschweig launched a separate investigation.
Hoyzer initially denied the allegations, but later admitted to them on 27 January. It was revealed that Hoyzer had ties to Croat organized crime syndicates who had bet large sums on the matches Hoyzer officiated. German media have reported that Hoyzer told investigators that he was present when other referees received payoffs from the syndicates and had heard that players were being paid as well. On 28 January, four arrests linked to the Hoyzer probe were made in Berlin; one of the sites was the café where Hoyzer allegedly met with his contacts. A Stuttgart newspaper also reported that the DFB president said players could have been involved in the fix.
On 2 February, prosecutors in Berlin announced that police had raided the premises of 19 suspects in the spreading scandal, and discovered evidence that may implicate as many as 25 people, including 14 players and three other referees, in fixing ten matches in 2004 alone. Hoyzer, who has been cooperating with the German authorities, provided much of the information that led to the raids.
On 10 February, the DFB announced that Hoyzer had been officially suspended for "unsportsmanlike conduct", after a hearing on 29 April he was handed a lifetime ban from refereeing football matches in Germany.
On 12 February, Hoyzer was arrested by police after new evidence emerged to suggest that he had fixed more matches than he had already admitted to. The German prosecutor's office suggested that he faced a long jail sentence if found guilty.
His information has so far led to the arrest of one other person, Dominik Marks, another referee.
On 24 March, the German newsmagazine Der Spiegel broke a story that expanded the scope of the scandal far beyond Germany, and possibly to UEFA headquarters. According to the report, Hoyzer told investigators that the gambling ring had the lists of referees and assistants who would work competitive international matches and fixtures in the UEFA Champions League and UEFA Cup about a week in advance of the matches. UEFA does not publicize the officials list until two days before matches. The story also noted that only the UEFA referee manager and the 11 members of its referee commission were supposed to have advance knowledge of these names.
The criminal charges Hoyzer admitted to carried a maximum prison sentence of 10 years, although there was no mandatory jail term. On 17 November 2005, in a surprise verdict considering his co-operation with the enquiry, Hoyzer was jailed for 2 years 5 months. At the same time Marks was convicted and jailed for 1 year 6 months, and betting ring leader Ante Sapina was jailed for 2 years 11 months.
Hoyzer's actions have led the DFB to adopt new policies to prevent further such incidents. The federation imposed a blanket ban on betting on football matches, effective with the 2005–06 season, by anyone associated with the sport—players, coaches, referees and officials. The DFB initially committed to assigning referees to matches with only two days' notice, but settled on four days' notice once it determined that its original proposal was impractical.
In June 2006 the Austrian art and theory group monochrom created an exhibition about the Hoyzer case. The project dealt with the relationships between fraud and "business as usual", between corruption and commercialisation.
In the appeal against conviction and sentence, begun on 28 November 2006, in Leipzig, there was surprise when the state prosecutor, Hartmut Schneider, found that although Hoyzer had cheated, he had not committed a criminal fraud, and called for Hoyzer's conviction to be overturned. However, on 15 December 2006, the court confirmed both the conviction and the sentence.