Upon deeper analysis, I found that the problem lies elsewhere. If I delete a series using the "Series" menu, this item disappears from the list, but not all items in the database. As soon as I rename some files (or replace them with a file with a different extension, for example, or change the diacritics) and add the series to the database again, some records remain original. Only some, I haven't come up with any rules. That's why the attempt to play the file ends with an error. Because I was replacing old AVI files with HEVC encoded MKV files, I mistakenly linked it. So the error is in deleting and adding items in the database.