Feedback: I run the 0.12.3 AppImage on Linux Mint 20.1 and don’t get this error. My installer is also 0.12.3.
You aren’t—by chance—trying to run Obsidian as root (a bad idea)?
My /tmp of course is owned by root, but my .org.chromium.Chromium.xxxx folder(s) and the files by my user.