MCreator issue tracker has been migrated to GitHub.
Existing tickets have been migrated to GitHub. Please open new tickets on the GitHub tracker instead. If you do not have a GitHub account, all feature requests and bug reports from forums are still considered and checked by the maintainers.
Crash logs, screenshots, anything?
Also make sure you don't have any 2020.2 plugins.
If this does not help, try reinstalling 2020.3 and reboot computer before running the uninstaller.