I ran into the same error message (“XML error: no element found at line 1”) and in all seriousness, it was easier to just go into the database & add the links there.
here’s the SQL statement I used:
INSERT INTO tablepreface_links (link_url,link_name,link_category) VALUES ('https://example.com','The Example Site',2);
a little find & replace rewrote my attempt at an OPML file into SQL.
(this whole OPML thing was a PITA! I was trying to get a handful of links from an unordered list in html into WordPress, and OPML was a dead end.)
