Can't join on my friends server.

I have this problem that occurred few days ago, where when I try to connect to my friend's server, it says after some time "Object reference not set to an instance of an object" in the command's. No one else has this problem my friend's server and we have tried many things to solve this; for example resetting my server character. This worked for one time, but now the same problem occurs. Thing is that this problem doesn't occur with other servers. Have this problem been reported to you before and do you know what would be the problem?