I had the same thing – a really wet/ muddy ride at Afan caused my RC41 XCAMs to seize. First time they've ever done it in 3 years of ownership.
The threads at the bottom are tapered, so will need a tap with a hammer to release the sliders. Use a rubber mallet or you'll wreck the thread.
I found re-fitting the sliders a complete pain, as the seals kept getting caught, and folding over themselves as I pushed the slider back on. About an hour of repeated trying and swearing seemed to do the trick.
Cleaning them thoroughly and lubricating the foam seal with fork oil has them working as good as new.