Legacy and Aether 2 with any diffriends mod lopping portal to infinity teleportation. Any portal (vanila too). If you enter the portal, they will teleport you to their world and immediately return you back to standart world then again to their world and so on. I think this is an important question, because with this error you cannot use any mod with an additional world. On the SMP

I have the exact same behaviour, we are playing this modpack https://minecraft.curseforge.com/projects/roguelike-adventures-and-dungeons

We use the following versions of aether:
aether_continuation-1.12.2-v1.2.3.jar
aether_legacy-1.12.2-v1.4.4.jar
lost_aether-content-1.12.2-v0.0.3.jar