I am using the X Mod Pack 2.0.1 (http://xmodpack.weebly.com/) which works just fine but as soon as I install Millénaire 3.3.2 (http://millenaire.org/) the game crashes on startup.

 

The Forge version in use is the one that comes with the X Mod Pack (minecraftforge-universal-1.4.6-6.5.0.472.zip)

 

ForgeModLoader-client-0: http://pastebin.com/rNZ272FJ

You have an ID conflict.  Millenaire wants to use slot 1515, buildcraft is already using it.  Open the millenaire config, and change 1515 to something else.

You have an ID conflict.  Millenaire wants to use slot 1515, buildcraft is already using it.  Open the millenaire config, and change 1515 to something else.

 

That fixed it but now the game itself is very unstable. When I am ingame (I created a new world) the game crashes like every 5 minutes.

 

new ForgeModLoader-client-0 part 1: http://pastebin.com/EHuFKZhY

new ForgeModLoader-client-0 part 2: http://pastebin.com/wfaNzdtT

Basically, the issue with you're dealing with is one of your mods trying to do bad things with threads (to put it simply).  I can't tell which mod id is from the error, but, I would suspect it's millenaire.  Trying removing it and see if stability improves.

Basically, the issue with you're dealing with is one of your mods trying to do bad things with threads (to put it simply).  I can't tell which mod id is from the error, but, I would suspect it's millenaire.  Trying removing it and see if stability improves.

 

I disabled Millenaire and now it all works just fine again. Too bad since it looked like such an awesome mod. But thanks anyway you have been a great help!

No problem!  I would suggest you report this bug to the Millenaire developers, as it could help them to fix this error.  And, yes, having played Millenaire in 1.8.1, it is an awesome mod!