Impedance-based stability criterion is a valuable tool in analysis of grid-connected systems. This work has shown a practical approach and a case study in which the stability criterion was applied to explain the instability occurred in a new data center in Southern Finland. The instability was observed as a sustained high-frequency resonance that had amplitude of 31% of nominal grid voltages measured at the grid-coupling transformer. The instability originated from paralleled rectifiers used in power supplying that had total power capacity of 250 kW.