Hey there,
thanks for your reply.
Here are some answers:
1) operating system: Windows XP
2) service pack: SP 2, no pre-release service packs
3) paint.net: Release 3.22
Well, I tried some more tests and noticed that it's some kind of mouse sensitiveness. I can't drag & drop an item sized 25x25 px because the mouse pointer doesn't switch to an arrow. If the item is 35x35 px the mouse pointer switches from a hand into an arrow only when positioning the mouse exactly in the middle of that object.
Didn't notice that behaviour earlier :-(