Any clues on what I should test with the debugger? Just running with "-snes_type test -snes_test_display -start_in_debugger noxterm" the debugger reports a PETSc error:
MatSetValues_SeqAIJ(), Inserting a new nonzero at (7,0) in the matrix (7,0) is certainly within the allocated space, considering the call to DMDACreate3d() that I sent before. I'm suspicious that the -snes_type test is somehow incompatible with the DMDA, at least without somehow initializing the Jacobian.
