When I was using run_mlm.py script to train a RoBERTa model, I also had the issue of not being able to set max_seq_length = 512. Thanks to this information on max_position_embeddings, I had to set --config_overrides argument to “max_position_embeddings=514” when executing run_mlm.py with max_seq_length = 512. The error did not show up a few training steps in. My model training is still in progress.