C1 has a function to catch all errors and it writes out [Error] to the page with no other information.
What I do to find the error is open the C1 solution in Visual Studio, attach to the C1 process and then open the page. You will see where the error is as a result.
Also a further step is to put some extra logic in the function that handles the errors so you can have it logged somewhere useful.
An even further note - C1 often get's annoyed with non xhtml compliant characters. That has been the problem with the majority of our [Error] messages. Wrap things in CDATA or escape things like "&".
Hope that helps.