Hi all!
I just came across this list of efile error codes. About halfway down the page there is a search box. You can type in the error code number and it will tell you what the error relates to.
I have seen a lot of posts with error code problems and did not get any any helpful links or info. Very frustrating to get the return(s) done and not be able to submit to CRA.
https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/rc4018/chapter-2.html#wb-auto-4