Anders--

I have played banjo for nearly forty years, frailing, Scruggs style, Melodic (Keith) style, etc. I assume that it is your right wrist which is giving you trouble; if that is the case, it must be that you are getting too tense when you play. That happens to my left hand when I play the fiddle. If it's your frailing hand that's troubling you, you may have too much wrist movement. Try a moderately tight elastic wrist band, to reduce movement, and concentrate on moving the forearm instead. And relax.