It seems that LibraryThing has recently updated the URLs of their author pages away from the format that has been whitelisted for linking to MusicBrainz artists. These links have been turned into:

https://www.librarything.com/a/[numeric-ID]/[slug] 

where /a/ denotes author and the last part (the last '/' character followed by the slug which is derived from the author's name) can be ignored (ideally, this part should be cleaned up when these links get added to MB artists).

In comparison, this is how the old links (still accepted for MB artists) looked like:

https://www.librarything.com/author/[alphanumeric-ID]

Here the alphanumeric IDs were derived from the authors' names, as opposed to the purely numeric IDs in the new format. These old URLs are still valid as they still redirect to the new links. However, it has become virtually impossible to add new LibraryThing links to previously unlinked artists in MB.

Example:

https://www.librarything.com/author/amostori -> https://www.librarything.com/a/3801