Hello, I just created an account to reply to this post with a solution:
The same issue happened to me, a few months ago I played a lot, and (just today) started to play agin and faced the same loading screen issue.
I had to go to the KSP2 directory (default for me is C:\Program Files (x86)\Steam\steamapps\common\Kerbal Space Program 2) and delete my BepInEx folder and my CKAN folder (I used to do KSP2 modding and I now assume the issue was because of these outdated folders/mods)
These folders still remained there even after reinstalling KSP2, so just go in there and manually delete any folders that shouldn't be there upon the first install, then (if you still want to add mods), just use the new versions of all the tools
Edit: I know this is tagged in unmodded installs, but this is the solution that worked for me with the same message on the loading screen.
