What version of MySQL are you using? It probably doesn't like the sub-query syntax that's being used.
Try disabling modules, one by one, until the error message goes away. I'd start with the contrib modules because I can't find that query in any code on my test site.
That query is right out of the theme_uc_catalog_bestsellers_products() function in uc_catalog.module. You could turn off the bestsellers block from the catalog admin page to make it go away...
FWIW, it works fine for me, on MySQL 5.0.45. It is a syntactically valid SQL statement.
Joined: 11/05/2007