Randomly teleported when entering Nether

Hello,

I have a modded 1.12.2 server. About 3/4 of the times my friend goes through a nether portal (overworld > nether), he gets randomly teleported in the Nether up to thousands of blocks away from the portal. When he goes through the other way (nether > overworld) his player appears in the portal on the overworld side and he immediately leaves the game. When he rejoins, the game says he suffocates in a wall but his items don't appear. This has never happened to me. Any suggestions would be appreciated.

Quote from Tomtaru

have you checked the console for overload messages?

So sorry for the late reply! We didn't get a chance to play for a few days.

I went in the nether with my friend twice (so I could /tp him back if needed). Both times it didn't teleport him, but the first time I couldn't see him and any items he picked up appeared like they were coming to my player. The only time there was an overload message in the log was immediately after I entered the nether, and it didn't teleport me anywhere. I'm hoping the problem just decided to go away and won't occur in the future.

Thanks for your suggestion