Can't convert albums

I have the following problem: iTunes (12.7.2.58) won't convert my albums anymore. When I select one or more albums in iTunes to, say, create an MP3 or AAC version from an ALAC file, it now all of a sudden says "none of the selected were converted because they could not be found". Of course, the files are there, and iTunes can play them perfectly fine when selected. iTunes can even convert the very same files if I select the files individually, rather than selecting an album. However, I have thousand of albums and obviously it's highly impractical to select every song individually if I want to convert something. It used to be no problem, but all of a sudden it's not working anymore. Is this a bug or is there a setting I can change so it'll work again, or, if this won't do, is there another recommendable that can easily organize a huge collection of music and convert from ALAC to MP3 or AAC when needed?