Hello, I play BG2 online quite a bit. Be glad to help you if I can.
In this case the answer has already been given. You and all players must have the same version installed as the host has.
The host does not have the TOB patch installed. Tell him to get it here:
http://www.bioware.com/games/throne_...pport/patches/
If your version starts with 2.5 I believe this means you have SOA+TOB installed. That is the best base to use even when you are still playing the SOA chapters. Note that you cannot play online with someone who has only SOA (patch or no patch) if they do not have TOB.
If you have version 2.5 you should defintely install patch version .26498 -- which is the version that you have. The best version.