Problem connecting nodes

The following problem: After a certain amount of time, KAP (5.10) behaves strangely when I want to connect new nodes. When I try to connect node B to a new node C with the mouse, KAP always takes an older node A as the starting point for the connection, even though it is located somewhere else in the workflow. (However, connecting with Ctrl+L works.)

Does anyone else have this problem, and what could be causing it?