Usher recently ended a Germany concert early and has been forced to reschedule several European tour dates due to an upper respiratory illness.
At the Berlin gig on Thursday, the 32-year-old R&B singer stopped singing in the middle of the song and said, "I'm so sorry," after which he was booed by the audience. He later left the stage.
The concert had originally been set for Wednesday but Usher canceled it at the last minute after feeling sick and scheduled it for the next night, People magazine said.
"Usher is suffering from upper respiratory illness which is currently preventing him from performing a proper show," a message on his website stated. "After consultation with doctors, he has been advised to reschedule the upcoming dates in order to get the rest he needs to ensure a complete recovery."
Shows in Holland and Belgium have been moved to March 1 and March 7, while the new dates for the 32-year-old R&B star's concerts that were set to take place in Germany have not yet been announced.
"I have been very excited about my European Tour and I deeply regret having to reschedule the shows. I am truly sorry for any inconvenience I have caused my fans," Usher said on his website. "My fans are the best in the world and I am looking forward to getting better so that I can give them the show they deserve."
Usher is known for hits such as "OMG," "Love in This Club" and "Yeah!" and is set to tour Europe as well as Dubai, Australia and New Zealand through April.
Check out fan-recorded footage from Usher's recent concert in Berlin.