I was in almost the exact same situation as you whilst playing with my brother. After tons of detective work, we figured out a fix--at least for ourselves.
Note:
Doing this will remove any warp doors or rifts you've opened/created.
Try going into the world-you-were-playing-on's save folder and deleting the folder "DimensionalDoors." Then delete the files "DimensionalDoorsData," and "DimensionalDoorsDataOld." It's recommended that you back these files up beforehand, but give it a try, it might work.
Afterwards, just launch the game, load your wold, open the LAN server, and see if your friend/brother can join.
If that does work, be really careful with rifts in the future and only use them when necessary. If it doesn't work, you could try clicking the settings toggle (cog) on the technic launcher and setting the version back to 1.4. I doubt that'll work, but it's worth a try; otherwise, I've got nothin'.
p.s. This will not break the DimensionalDoors mod, it only resets it. If you don't really use the DD mod, you could alternatively just disable the mod altogether.