copy the geodatabase out to a new featureclass (Copy tool) with a totally different name
dump any unnecessary fields so that you just have the required fields (ie objectid, shape, shape_length, shape_area, shape_whatever ...)
do the calculation on a newly added field.
If that doesn't work.... start a new project... add just that featureclass and try again.
Report back since something is messed up an a clean reboot, restart with clean data is the only way you will track it down.