The Library of Congress did in fact switch their catalog to a new system in the past year or so, and I think some breakage is expected. It would be more productive to report this issue to them than complaining in a blog post they probably won't notice. They've been receptive to feedback in my experience. I reported a cataloging issue to them before and the issue was fixed. There's a link titled "Feedback" at the bottom of every catalog page.

Also, I'm not sure why the author is maintaining their own bibliographic database directly in Sqlite. Something like Zotero would probably be a better fit for them. (I'm aware Zotero uses Sqlite, that's not the point.) Zotero's bibliographic data importer works fine for the Library of Congress, even now.

Edit: I reported the bug to the Library of Congress myself:

> In the catalog, for seemingly every record, if "MARC record" is selected, "Download" is pressed, and any of "Dublin Core", "MODS", "MARC XML", or "Bibframe" are selected, a 404 error is returned. Those forms of cataloging records can not be downloaded and should be fixed. Thank you.

Edit #2: I got a reply within 5 minutes:

> Thank you for taking to the time to report this issue. We have noted the problem and are looking into it and hope to have a fix in a future release.

Edit #3: And rather than reply to a blog post in a place the author won't see, I've emailed the author about how he can report issues to the Library of Congress, and noted that I've already reported this bug.