This could be related to many things.
Are both systems HDX codecs and SoundStructure?
Does the problem only happen in one direction?
Is this happening on all of your HDX and SoundStructure systems?
Does it happen when you only call a room with just an HDX codec?
Are you using C-Link to connect the SoundStructure to the HDX codec?
Things to look at, AEC reference set properly, is there enough signal level at the defined reference point.
Is the gain structure in the system balanced, specifically the output level of the SoundStructure is properly matched to the local amplifier.
Another remote posibility involves a bug that was in older software for the codec but may have resurfaced.
See the attached document to test if this is related. If so this should be escalated with a call to tech support.