Probably either a 6i6 hardware issue, a driver problem or an issue with the cables. I've had 2 KPA/6i6 issues lately, both caused by USB or SPDIF:
1) I could not lock my spdif, where everything looked fine. Turned out, the spdif cables at the KPA side were a little loose. So it worked, but sort of half. Reseating it fixed it
2) From the 6i6 I got huge white noise, or rather more like digital overdrive. Reseated the USB cable into a different USB slot in the PC and it was solved.