Why isn't Itunes importing MP3 files?

So I've been using the YouTube to mp3 converter sites for years to get unofficial soundtracks and other music that you can't find in the Itunes store into my library. Ever since 2010 I've been readily able to import any mp3 file to Itunes. Since when did Itunes stop accepting MP3 files, and since when did downloaded content get restricted? From what I can tell, any attempts to add any local files results in Itunes not accepting the file. Converting it to MP4 allows it to be added and converted or named, but as soon as you attempt to play the file, Itunes crashes. I'm very suspicious of this sudden behavior. I will leave Itunes for a better service should this issue be unresolvable. MP3 conversion and use via local files for my library is a must. I've seen some answer circulated that it's just "some file format Itunes doesn't like" That's not an answer and tech ought to know what files their own service is capable of using and that should be clear. Any help is appreciated. If you can't provide a way for me to change the settings for importing and I can no longer use itunes to manage my local music then I will find a better service.