The url must start with http:// (or
ftp://, mailto:// etc.) or else it is interpreted as relative and the link is prefixed with the current location. When you save an entry with a url field, the http:// is automatically added if the protocol is missing from the url. Apparently, when you import entries, the module doesn't add the http:// where omitted, and all the imported urls which do not contain the protocol will be relative and broken if intended as absolute. That's a bug in the
Database import and is worth reporting in the tracker if not already. Workaround: before import fix the urls.
hth