Perhaps there is an issue on Last.fm's side as well.

It seems that if the username's case (whether the letters are uppercase or lowercase) is incorrect, it results in a 500 error.

My username is 'Rowolta', not 'rowolta.'

The broken link looks like this: https://www.last.fm/user/rowolta/library/music/Misery%20Index
The correct link should be: https://www.last.fm/user/Rowolta/library/music/Misery%20Index

A solution could be to use the username provided by the API to ensure the correct casing?