The game asks me to enter my serial key, I have found it on the gog website I copied and pasted it but it doesn't work.
They ask me to put a key in this format XXXX-XXXX-XXXX-XXXX, but then one I have is 6 digits longer, should I contact the GOG support or am I missing something??? It started happening after i tried to mod it, it says that its an incorect serial key. I even tried to do the manual activation but the key they gave me is too short. If it helps I bought a key to the game.

Are you looking in the right place? Access GOG.com through your web browser. Hover over your name and click on 'Games' on the pull down menu. Navigate to Warband and click on 'More' to pull down a menu then choose serial keys.

I do not have Warband so do not know if it is broken for that game.

I have the same problem, my key is 24 characters long.