The server/host would need to start the server with xmax and all players that want to join will need the same version of the mod (at least this way it worked with the original Titan Quest/Immortal Throne version).

Joining an online game or even a LAN game is a bit confusing with the x mod, however, once you realize what's happening it makes sense.

Have someone host, they do that by clicking Custom Quest, selecting the map and then clicking start. You then get the error about bounce, however that has officially "initiated" the mod. Now they select again LAN or Internet and create the game.

Now comes the somewhat confusing part. The joiners also need to follow that first step which is to say Host Lan or Single Player in order to "initiate" the mod. After they get the bounce error, then they can actually select join multiplayer and they'll see the server list and be able to join without mismatch errors.

I got a crash right after creating the game ...
Anyone know why ?

same here, have crash, when trying to create game

I finally found why I made the game crash. You don't want to launch the game as MOD quest gameplay. You just want to activate it :

So launch the game
Go to personnal quest
Select the XMAX you wanna play until it bounce - You are now into XMAX until you relaunch the game -
Go back into the main menu
"Play Titan Quest"
Set multiplayer HOST / JOIN
And it's done ... Check the number of monster at the horse quest.

*Push* Same issue, different user. Floppy's advice didn't solve the problem. Any other suggestions?

Host: Personal Quest -> xmax type u want to play -> Play Titan Quest -> Host server
Others: Personal Quest -> xmax type u want to play (must be the same as the host) -> Play Titan Quest -> Join Server

No problems for me

nice mod for single player sadly doesnt work on multiplayer , not for me or my friend

bot did the basic setup
start it in personal quest du initiate the mod
then go to play titan quest and HOST a game
the moment we try that it just gives an "connection problem" and closes the game a few seconds after

hosting without the mod , works tho

king of late answer but just to be seen by people who might need help: it worked on multiplayer for me bouncing the mod on single player. That is:
You start an xmax map game from custom quest on single player. Mod is bouncing
THEN u host multiplayer or join multiplayer game from the game screen.