Thanks for the answer. Yes I've been able to do that. But I like to do it for all my feature classes (that have the field SHAPE_area) at once, because I have so many of them. Is there a way to do that? Editing orig. question to clarify.

@Swingley: Thanks for the tip. I just tried both Check and Repair Geometry against the feature class in question. The problem remains.....

Just tried to right click on the "SHAPE_Area" column (both on the column label and in the column), but the "Calculate geometry" (and "Field calculator") options were also greyed out! Any other ideas? Thanks.