Um, I'm not sure how to do that
Are you using Windows? If so, using your Windows file browser go to This PC -> Local Disk (C:) -> Users. In that folder you should see a folder named after your Windows username. Inside it should be a file named demonshymn2_log.txt. If you're able to find it, please attach it to an email and send it to bugs@untiedgames.com.
Otherwise thanks for the bug report- Even if I'm not exactly sure what the problem is, I'll try and add some backup method of drawing in the next version. Thank you!
In that case, the log file should be in your Home folder. You can find it by opening Finder and pressing Command-Shift-H.
Without the log, I can only guess what the problem is... The game has had very limited testing on Mac, but it does work for my tester. I'll let you know when the next version is out!