Please check the XLR cable(s) you are using. We have seen that some cables (i.e. the cable attached to headset microphones*) where the XLR-housing is connected to pin 1. The solder tag for the XLR-housing is typically labeled "G" and connects to "Chassis-Ground" Pin 1 on the XLR plug is "Audio-Ground" These two different grounds should not be connected. Please refer to the picture below. The cable shield must be connected to pin 1 only.
* Note: i.e. the APEX-270 headset mic.