Previous studies on the neurophysiological underpinnings of feedback processing almost exclusively used low-ambiguity feedback, which does not fully address the diversity of situations in everyday life. We therefore used a pseudo trial-and-error learning task to investigate ERPs of low- versus high-ambiguity feedback. Twenty-eight participants tried to deduce the rule governing visual feedback to their button presses in response to visual stimuli. In the blocked condition, the same two feedback words were presented across several consecutive trials, whereas in the random condition feedback was randomly drawn on each trial from sets of five positive and five negative words. The feedback-related negativity (FRN-D), a frontocentral ERP difference between negative and positive feedback, was significantly larger in the blocked condition, whereas the centroparietal late positive complex indicating controlled attention was enhanced for negative feedback irrespective of condition. Moreover, FRN-D in the blocked condition was due to increased reward positivity (Rew-P) for positive feedback, rather than increased (raw) FRN for negative feedback. Our findings strongly support recent lines of evidence that the FRN-D, one of the most widely studied signatures of reinforcement learning in the human brain, critically depends on feedback discriminability and is primarily driven by the Rew-P. A novel finding concerned larger frontocentral P2 for negative feedback in the random but not the blocked condition. Although Rew-P points to a positivity bias in feedback processing under conditions of low feedback ambiguity, P2 suggests a specific adaptation of information processing in case of highly ambiguous feedback, involving an early negativity bias. Generalizability of the P2 findings was demonstrated in a second experiment using explicit valence categorization of highly emotional positive and negative adjectives.