As the title says, whenever I try to load certain worlds the error pops up and leads me to a multiplayer server despite it previously being singleplayer. Curious enough, this only seems to happen if a world is last saved in survival mode. I have no such issues on the iMac, purely WIndows 7 now.
I have these mods installed:
Mr. Crayfish's Fruniture Mod
I also have NaturalRealism resource pack as well as Sildur's Shaders.
All I see is it tried to join you into the world for 3 minutes and then timed out, which is reasonable, if you can't join a world after 3 minutes it has to assume something went wrong. Try without resource packs and shaders.
Oh okay! That makes sense. Because after a few tries, I'm still able to get in. And admittedly it does work better without these things. Still, is there nothing that can be done to minimize the problem.