The initial release of macOS Catalina caused trouble for a lot of Little Snitch users. Updating an existing version of Little Sntich would almost always lead to a dreaded “Version Mismatch” error being shown after restarting your Mac. We documented our findings and contacted Apple about this issue.
Thankfully, the macOS 10.15.1 update released today fixes the issue. After installing this update, you can install and update Little Snitch and everything should work as expected.