Workflow path silently gets ignored

Proble

In a workflow run a task does silently not get executed even tough one  task link fulfils its condition.
task_correctly_not_run(1)
                         \(a)
                          task_should_have_run(2)
                         /(b)
          task_had_run(3)
                         \(c)
                          task_that_must_not_run(4)

Solution

For tasks (2) of which one input link is not trodden (a) and for which the input links (a, b) are connected with AND, none of the link conditions (a, b) gets evaluated but silently discarded even tough there are predecessors that have run (3). Possibly it is wrong to use AND but one should use OR instead for the link connection in (2).