Complex sentence: Some doubts remain as to whether a proof spread over 500-odd articles is complete and correct, and these doubts were justified to a considerable extent when gaps in the argument were found, although all gaps known to date have been fixed.
Split into simpler sentences: Some doubts remain as to whether a proof spread over 500-odd articles is complete and correct. Some gaps in the argument have been found, although all gaps known to date have been fixed.