Tuning problem

Hi folks! I ran into a problem last night that I hope someone here might have some insight into. I play a Kala soprano ukulele, and always use an electronic tuner because I'm lousy at tuning by ear. Last night, I noticed that on my C string - and only the C string - the open string is in tune but any fretted notes are sharp. I'm hoping it's just because I've had these strings on for a long time (too long?), but I wanted to see if there was anything else I should be looking at. The neck doesn't look warped in any way, and the problem is just with this one string.

I'd try throwing a new string on, let it settle in, and see if you still have the problem.

I once had intonation problems with a single string. A new set of strings sorted it.

When strings get nicks and/or wear out this will happen.