News agency dpa reported Saturday that Wladimir Pritzkau, the deputy head of the Evangelical Christian Baptist congregation involved, as saying the service took place on May 10.
He did not say how many people were affected but said that most are at home and six are in hospital.
Religious services have been allowed in the region since May 1, with conditions that include a 1.5 meter (5-foot) distance between worshippers and the provision of disinfectant.
Pritzkau said rules were adhered to. The church has canceled all gatherings and is now conducting services online.
The head of the city's health office wouldn't confirm or deny the case, citing medical confidentiality. (AP) CPS