We also received an email from you about this and replied but also wanted to reply here, in case you didn't receive our email message back. Anyway, here's the deal:
Getting an “out of memory" error can mean that the application is having difficulties saving your Accounts document because some unexpected issue is occurring behind-the-scenes. Would you help us gather some details so we can piece together exactly what's happened there? To that end, the first thing we’d recommend is that you download a new version that's available at our website that will help with troubleshooting. Here’s the link for where to learn how to get that new version installed (even though the page discusses El Capitan compatibility, using that new OS isn't required or involved with this issue):
http://www.splasmata.com/?p=2952 .
After getting the new version installed and launched, if the issue persists, would you be willing to send us your recent system logs to
support@splasm.com so we can take a look at any error messages logged by your Mac or other processes around the same time as the error in CheckBook? To gather these logs, open CheckBook, hit OK on the error message if it still shows, go to the Help menu while holding down the Option key on your keyboard, click the Go to System Logs Folder menu item to be taken to the logs folder, then send each item whose name begins with the word “system”.
Keep us posted on here or through email!