Mysterious illnesses occurred during a performance at an opera house in Germany.. What happened?

A number of attendees at a concert were treated by paramedics on Tuesday, after suddenly feeling unwell during a performance at the Semperoper Opera House in Dresden, Germany, before the same feeling spread to medical workers as well.
The Dresden Fire Department said in a statement that some attendees reported circulatory and respiratory problems, and two of them were taken to hospital.

Respirators

Four paramedics later felt similar symptoms, prompting the fire brigade to call in specialized units equipped with respirators.
The paramedics were examined by a doctor, and the police reported that one of the members of the public was unconscious, but no harmful substances were discovered in the building.
The reason behind this is still unclear, while the police have opened an investigation into the matter.