This has been logged with WD support, together with an extended log.zip. and the support website states it is still under warranty. The unit has recently (after a previous freeze) been given a complete strip, clean and blow-out with compressed air which included popping the drives.
The only information the logs suggest is this occasional failure to read a random drive temperature. All the S.M.A.R.T. info appears to be as it should.
There is a large, old and ongoing thread on this problem in the EX section, but I thought I would post here to see if any DL users had come across similar issues.