Thanks for the answers. It actually good to know, that this is not a common issue, so there is a hope I can fix it at some point!
I have a chief medical dwarf, and a hospital (with beds, traction benches, crutches, splints, and everything). The chief medical dwarf has dabbling diagnosis skill though (I have no better among others). So, if a bad diagnostician can fail to see health troubles (I didn't know that), this defenitely can be an issue. Yet, none of dwarves requires diagnosis, neither they go to the hospital. Even the chief medic himself always crawls and has no worries about it.
I don't have any evil fogs (biome is not evil), and I didn't touch a single forgotten beast in this fortress. But I actually like the idea that it can be some syndrome brought by the immigrants. I play the series of fortresses, so more than half of the dwarves came from the earlier fotresses, where they possibly could be diseased. But the syndrome typically spreads to many dwarves very fast, isn't it? Here the number of affected dwarves seems to be quite constant.
What does their v->w screen say? Does it say prone? Climbing?
It says nothing. But it always does so, isn't it? Sleeping dwarves are also prone (they have the same icon background also), but v-w says nothing about it.