KAMELOT backing singers Elize Ryd (also of AMARANTHE) and Alissa White-Gluz (also of THE AGONIST) filled in for NIGHTWISH vocalist Anette Olzon last night (Friday, September 28) at the Ogden Theatre in Denver, Colorado after Olzon was taken ill and had to be hospitalized.In a posting on her official blog, Olzon wrote earlier today, "I am so sorry that I couldn't be on stage yesterday in Denver. I got very ill the night before and first went to the ER in the morning and [was] there for five hours before they released me with some painkillers and thought it was a kidney stone. Unfortunately, I got worse at 7 in the evening and started vomitting heavily and my fever rised. After talking to the hospital, they said I should cancel the show and come straight back in to the ER. I was there all night and I was very dehydrated so they gave me 2 litres of fluid though an IV, they did blood tests, ultrasounds on my kidneys and finally a CT scan. These tests didn't show why I was so ill, but probably the stomach pain has a connection with me having the flu or just stress. They did find something else, though, and for that I'm grateful. If I didn't do the scan, I wouldn't know there's a big cyst in my liver and I need to go check it out [as soon as possible] at home. "I do feel better today but still fever cough and a very sore throat, but hopefully I can be on stage tonight, even if it means shorten the set list." Fan-filmed video footage of last night's concert can be found at this location. NIGHTWISH and KAMELOT are scheduled to perform tonight (Saturday, September 29) at The Complex in Salt Lake City, Utah. Olzon joined NIGHTWISH in 2007 as the replacement for Tarja Turunen after the latter was fired in 2005.