WordPress database error: [Can’t find FULLTEXT index matching the column list]

When I write a new post and save I get the following errors multiple times (only showing a small subset below):

WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT MATCH(post_content, post_title) against (‘worldwide’) as score FROM wp_posts where id=216;
WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT MATCH(post_content, post_title) against (‘success’) as score FROM wp_posts where id=216;
WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT MATCH(post_content, post_title) against (‘discuss’) as score FROM wp_posts where id=216;
WordPress database error: [Can’t find FULLTEXT index matching the column list]
SELECT MATCH(post_content, post_title) against (‘variety’) as score FROM wp_posts where id=216;

But the post saves OK.

Recently the Manage Posts screen started showing only one post. None of the other posts show on this screen and I can no longer edit them. The posts are still available, they just don’t appear in the Manage Posts screen.

If someone can help me with this problem I will appreciate it.