ok, this is a little strange. Going through my tests one of the procedures is to
SAVE SLD the currently just put together. (Test: SECTStudio013) In this test we
build two classes one with no expression and a second with a simple expression
for a polygon feature type. 

Once saved, SECTStudio is reloaded and the context and the new saved SLD is
reopened. Both Context and SLD can be previewed.

Once going to the Classifer, the symbol styles, the Label styles and label
attribute where properly loaded.

What changed was:
1) There was no longer a expression attribute selected: ["NA3DESC"]
2) No class names were saved, and not found in SLD layer styles.
3) The class with the expression had changed from a simple expression to a
logical one. eg. From "Africa" to ("[NA3DESC]" = "Africa")