I've been trying to go to my friend's server for months, but couldn't as it would say 'Unable to connect to world'. My friend and I had tried the solutions found online, such as needing to sign off then log in again. We've even discovered that it is specifically my minecraft and not theirs since they can still enter other people's private worlds. 

I would just like to know to fix this problem personally from the creators of this game. Thank you for your understanding.