Obviously this monitor is not factory precalibrated. So the calibration probe did a great job.
Look at the final color correction diagram. Green color was corrected most of all.
The Dell 2408 achieved the best value when calibrated to AdobeRGB.
The result is astonishing. Several separate sessions were done with the use of two different computers and three different programs. Results varied minimally. dE values in all cases did not exceed 1.0
The best result achieved: dE=0.4(!) – near to perfection for appropriate (color gamut) content.
The reference monitor has dE=0.2
Calibrated Dell 2408 displays smooth gradients.

Optional calibration was done at ~ 140cd/m2 of white with brightness control set to 7-8, contrast 50%, R = 96, G = 92, B = 100. This is by no means a universal recipe as models vary.

Not the best news is that sRGB preset mode failed with dE=7.3. Calibration did not help. To be honest, I did not expect much from sRGB mode on this monitor.