Given that it previously worked, it means that the element that used to have an id of "C1" on it is now no longer available.
There can be a few different reasons why this might occur:
- The element used to be there but has now been removed
- Something has happened to cause the element to be added to the page later than the code which needs to access it
- The code that needs to access the element runs before the element has been created
How I would approach this is to set a breakpoint on the offending line, and investigate the HTML and scripting code to find out what clues might be divulged from such an investigation.