Thanks so far for the replies. It's only two cacti that have this problem. I'm growing around 50-60 now with some more on the way. All other's are looking fine, like little spiked green thumbs sticking out.
I've managed to make a few better pictures. Following advice here, I've put the 2 fts which provide light a little higher and I put the two cacti back in a terrarium with only a fluorescent bulb and high humidity. If the light was causing the problem, I wonder why some of the much younger ones which where under similar conditions didn't show any signs of trouble.
Something I forgot to mention, I've added 1/10 of cacti fertilizer to all cacti older than three months. Could that be the problem? Overwatering does not seem a problem, the soil was dry and I water twice per week.
Anyway, they seem a little better than before but maybe someone else has an idea seeing these new pics.