Why does the server turn off while converting to 1.15?

So, my server was previously on 1.12.2 and worked fine, till I decided to upgrade it to 1.15.2. Now everything was still working, but really lagging. I thought about the "Force Upgrade" option in the worlds page. When I activated it and started the server, it seemed to be working. But after about 5 minutes, the server just went off. Is the server time out still active while converting? And when yes, why? Is there another problem with the world? I don't have any idea.