Some players are invisible in multiplayer

Hello,
I recently made a server to play together with a family member, so I made it offline (doesn't connect to mojang), everything went alright, except the fact, that the other player can see me, but I can't see him. I suppose that it could be, because I have the default Alex skin while he haven't changed his skin for a very long time.

The picture doesn't tell much, but he's standing approximitely at my cursor's position