When you run FM 0.6.5 for a long time (three days in my
case), it forgets how to grey the active node. You can
still edit and everything but the node under the mouse
won't be painted grey anymore.
When you create a new map, copy nodes or open the
help, then the painting works for those maps but not the
one in which it didn't work.
Restarting FM solves the problem