Failed with connection

Hey,

me and my friend were playing on server that i have hosted on ma computer. The server has caves included. We have survived many days, but when we wanted to come back on server 2 days later, we couldn't. I can host the server but my friend cannot connect to me. We tried to make new server and we find out that the problem is in the caves. Once caves are added he cannot connect to me. When caves were not added to server, he can connect to me without problems. We didnt change anything in our game or client.