This is really odd, so sorry if I ask stupid questions.
1. When you interact with it what button do you use? You should click on it with the, by default, left mouse button.
2. Any chance you have entered the code already but forgotten about it
Try and interact with the door and see what it says.
3. And you have tried playing from the beginning again and it still happens?
4. There is no chance that maybe you have reconfigured the keys since entering the second part of the mine, making the interact assigned to a new key? you can for example still pick up rocks and so forth?
If answer to the above is yes then
go to My Documents/Penumbra Overture/Episode1/ and delete the settings.cfg file, play the game from start and see if that helps.