Minecraft multiplayer problems?

So far we have not found what we're looking for on the Internet, so we're trying to do it this way. My partner and I play Minecraft Multiplayer over LAN, which has worked without problems so far. Recently we have had the problem that as soon as someone joins the server, the game is extremely buggy. You can't use items, break down blocks, take in food, monsters do not react etc.

We both have the latest version of Java; 1 to 1 the same mods (and none were added) and we logically play Minecraft on the same version. The game was restarted several times, but it didn't work either.

Does anyone know why and what you could do about it?

Do you play on a server that is hosted on the PC or on an external one? You are in the same lan or separately. All mods up to date? More information please.