Could it be that in Dynamic Topology, you Detail Refine Method is set to Subdivide Edges?
If so, change that to Subdivide Collapse.
Oh Right!
It works fine now. Thanks a lot!
However, can you spare a few minutes to explain what subdivide collapse is doing different from Subdivide edges ?
Actually, I can't, because I don't really understand it;)
There are three modes as you can see, Subdivide edges, which subdivides long edges to make new geometry.
Collapse edges which collapses short edges if they get shorter than the resolution you have set in Dyntopo.
And Subdivide Collapse, which does both.
I usually leave it at Subdivide Collapse, unless with creasing...
Kent explains this better than I can, if you continue watching;)