As reported by mrdeadlocked on irc: http://pastebin.com/FHBkkeAv

• just logged in
• player is in overworld
• chunkloading mode 2

mrdeadlocked: If i let my base completely unload the chunks. (Watch them disapear on opis) then wait for DIM3 to unload (because i haveoption 2 set), when i /home to go back to my base (which is entirely unloaded at thsi time) server crashes

Was an issue with cauldron unloading the dimension.
Resolved by changing the cauldron config accordingly.