"Headers already sent" after uploading to website to server

[eluser]toki[/eluser]
Hi,

i developed a codeigniter website on my local server (apache, ubuntu). It works fine.
Now i uploaded it to my webserver. I get the error "Cannot modify header information - headers already sent by"

I understand that this error occurs when some output is printed and a redirect is done afterwards. But how can there be a difference depending on the server the site is hosted on?

And what is the best whay to find the source of the error?

Thanks for your help in advance!

Toki