This concerns one specific score by @Mike Magatagan namely the score https://musescore.com/mike_magatagan/scores/3004231
If you click "Download" and choose "PDF including Parts" the returned document is not a PDF but a "Not Found" error in XML format such as:
<Message>The specified key does not exist.</Message>
All other export formats work fine.
As a test I've downloaded that score in mscz format, opened it up with musescore 2.3.2 and used "Save online" to save it privately into my account (private url https://musescore.com/jeetee/scores/5304581 ). From there I can download the PDF with parts without issues.
Mike already tried to "update" his score by resaving the score to his account; we were hoping this would force the musescore server to regenerate this PDF. Alas this seems to not work.
Can someone on your end ( @Ximich or @abruhanov probably) debug this and/or force the server to generate that file?