I have too much 404 errors, how to learn the source

As you can see from this article, the source of 404 error may be different. MijoSEF offers you a method to learn the source.

Go to MijoSEF=>Configuration=>URLs=>Log 404 errors option and set it to Yes. With this option MijoSEF will store the 404 errors in a log file found in yoursite.com/logs/mijosef_404.log including various info such as referrer about the 404 error.