Well, it looks like my HE400 has been "acting up" these past couple of days. The left channel appears to randomly cut off and there also seems to be some from channel imbalance from the left channel at times. I purchased it in June 2012 from an Amazon.de Marketplace vendor, and up until now it has been performing very well.
Anything I should try before getting in touch with HFM?
*By the way, the cable appears to be undamaged, and the rest of my setup has been functioning flawlessly with other headphones*
Make sure the cable is screwed in properly. Do some counter clockwise turns before screwing it in to take some of the tension off.