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.