Having the exact same issue. I’ve been trying to check in for hours, without any luck. Prompt says “Checkin for assets to check in” for a few minutes, then it goes away, and then nothing.
Also, yesterday, I also had issues with ctrl+z and saving the project, making me lose hours of progress. And I also had the issue of UEFN crashing because it got stuck on the ‘Checking for assets to check in’ while eating up all of my RAM. Extremely frustrating.