They have to make sure that the UICC number on your account matches the one on your UICC, or you will have that problem. Should be easy enough if you get someone decently trained on the phone. I found that the overseas reps keep confusing it for an SD card and can't figure it out, but the American reps can do it easily. I know this is the issue you are having because I tried to use the UICC from my other Sprint HTC One while waiting to get a replacement for the defective one my phone shipped with, and ran into the same exact thing you are seeing (stock ROM too).
Worst case scenario you can take it to a Sprint corporate store. Mine gave me a replacement UICC for free and changed the UICC on the account. (Story for me is that mine wasn't being detected at all, bought phone from Wirefly and Wirefly wanted me to send back the whole phone, Sprint refused to mail me a UICC, really didn't want to send phone back so I went to corporate store instead).