Please check the troubleshooting guides before posting!

[Romulator]

This is a custom story issue, moving to Custom Story Tech Support.

StringToInt is code used in Amnesia versions 1.3 and above. Provided you are using a Windows, you can find updates to Amnesia here: http://www.frictionalgames.com/forum/thread-24334.html

Otherwise, use your respective Mac or Linux store which you downloaded the game to update.

If the problem continues, you should attempt to contact the developer of the Custom Story you were playing. Depending on where you downloaded the custom story, there may be different ways to contact the developer, such as Personal Message or a comment system.