I explained it in the post above. Just entering the coherence is not a valid logical expression. In this field (valid pixel expression) you define at which locations your result is usable. As coherence is a measure for quality of the interferometric product you only want those pixels at a location where the coherence is larger than a threshold, let’s say 0.4.
Thus, just putting the coherence there is pointless because the expression is lacking a condition. Enter this instead:
coh_23Nov2012_08Feb2013 > 0.4
Then you will have bad quality pixels masked out and only the “good” ones remain.
Depending on your colour scheme it should somehow look like this:
As your temporal baseline is quite long (about 75 days between first and second image) coherence values could be quite small.