Hello there,
what worked for me was that i went into the file manager and deleted the folders "world", "world_nether" and "world_the_end", make sure that your server is offline when doing this. After that your server should start up and work.
So basically my friends server has been just not working at all. When we start up the server and we join just the whole world is missing, just a blank space or we join and only around 10 blocks around us spawn and if we go further we just fall into the void. Then after around 30 seconds it just kicks us with the message, Could not connect to a default or fallback server. And after this message we aren't able to rejoin until we restart the server just for the same thing to happen over and over. This has been going on for 3 days and we don't want to create a new server as we have so much in this server such as gigantic builds. Someone please help.