Unfortunately, Version 1.x isn't guaranteed to work perfectly on the latest operating system—we haven't released any updates to that older version since the release of version 2 several years ago. The thing about the update checker is that it's looking for 1.x updates, of which, 1.8.1 is the highest available. To get on a version that's definitely Mountain Lion compatible, you'll want to upgrade your license and download the latest version. For that, you're eligible for a 40% off upgrade discount on either CheckBook or CheckBook Pro so, if you're interested in going that route, shoot an email to
support@splasm.com and we'll send over a coupon that works at our online store.