I've only seen 2 reports of this problem so far, one of which was mine. This error seems to be caused by failure to load the UI core, because every time it happens, MCreator's loading screen ALWAYS gets stuck on "Loading UI core." I have seen this error in 2020.4, I haven't seen it 2020.3, but I did run into it in 2020.2 not too long ago, after having seen it in 2020.4, although you would only really care about 2020.4. A better description of what happens is that when the UI core fails to load, the MCreator window renders as a blank white window, hence the name I have given it. In this window, you can still interact with things, but every window will be white. I have no idea how to fix this for those who are having this error, but hopefully we can find a solution.
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.