'external clock choice is invalid' message

[nplmdthfan]

First, I'd like to thank those who chimed in on my previous problem (audio clicking on the lightpipe input in mid-Novembre). Your help was invaluable, cheers for that. But now I've got another problem.

I am having a problem getting audio into my ADAT lightpipe input. Everything worked fine just a week ago, but now nothing. I am going analogue into an SM Pro Audio A-08 analogue-to-lightpipe converter. The lightpipe goes into my Digi002R. I then go to the HARDWARE SETUP. Digi002 is the only 'peripheral'. My digital input is set to 'RCA = S/PDIF, Optical = ADAT'. I then set the CLOCK SOURCE to ADAT and click 'OK'. What I get is the following:

The external clock choice is invalid. Reverting to internal.

Obviously I'm overlooking something. The sample rate matches at both the converter & PT, so that's not it. I've also changed my lightpipe cable from a cheap Hosa to a better one made by Monster, thinking I may have damaged the Hosa when I pulled it out a week ago. In fact, that's when this problem started. And, as far as I can tell, I haven't hit any 'wrong buttons' on my keyboard. Any hints out there?

Cheers,
nplmdthfan