Green Day frontman Billie Joe Armstrong has been discharged from the Hospital Maggiore in Bologna, Italy, after being rushed there Sunday to treat a mystery ailment that forced the cancellation of a Green Day concert in the city, reports W.E.E.N. No official cause of his illness has been given, but local reports suggest Armstrong was suffering from gastric problems.

Armstrong reportedly discharged himself on Monday, a day after what would have been the final date of the band's summer European tour. The Italian publication La Voce also reported that some fans caused disturbances at Hospital Maggiore by attempting to visit the singer.

Green Day's next scheduled concert is the iHeartRadio Music Festival in Las Vegas on Sept. 21, although the band also is among the performers lined up for the 2012 MTV Video Music Awards, which takes place Thursday (Sept. 6) in Los Angeles.

As previously announced, Armstrong's bandmates Mike Dirnt and Tre Cool released a video apologizing for the cancelled show. "We apologize profusely," Dirnt said in the clip, which was posted Sunday. "This is really unfortunate, but Billie got rushed to the hospital last night due to illness and the doctors don’t think it’s a good idea for him to play today. This sucks."

No information regarding a makeup date or refunds for Bologna ticket holders has been announced.