When using an execute node, after the function being executed, godot throws the error 'Invalid get index 'next' (on base: 'Dictionary').' I suppose it is either a bug or me doing something wrong.

The problematic bit seems to be set_next(current_node.next) on the execute func. 

I managed to do the bit by using a condition branch and checking if the result was different then false. But execute seems to be bugged

Yes, the Execute node in DD was indeed bugged. Just updated DD to version 3.2.3. It should be fixed now.