Some worlds won’t let players join when using imported level.dat files. When the level.dat files are swapped out for ones that the server software made itself, even when keeping all the other files the same, everything works fine and players can join.

The reason I said “some worlds” is that I didn’t have this issue with another world I imported. I think this has to do with the size of the world, the smaller one worked fine, but I haven’t tested this.