0 fps in 1.15.1 vanilla server (updated from 1.14.4)

I just updated my 1.14.4 vanilla server to 1.15.1 by just replacing the JAR file and when I try to login, I get 0 fps and my game becomes unresponsive. If I kick myself (from the server's side), the game works as expected (well, until I login again). I tried creating another 1.15.1 vanilla server but I didn't experience this issue. Any help?