Print Invoice - Is there a difference between local and live version?

Question

I put my latest local version live. Now I have the problem that I can't print invoice pdfs anymore. Packing slips, orders and credit-memos work without problem. On my local version, there was never an issue.

error report tells me that

a:5:{i:0;s:172:"Cannot send headers; headers already sent in /bentoLunch/bentoLunch/app/code/core/Mage/Bundle/Model/Sales/Order/Pdf/Items/Invoice.php, line 1";i:1;s:1726:"#0

What is the difference in printing pdf on local server and now trying to print the pdf on live server? Maybe if I know that, I can fix this somehow. I already read a lot of posts, and I can assure you, there is no var_dump, echo or print_r inside the Invoice.php. There is also no empty char before the <?php.

And another thing. I send a pdf invoice via email after purchase goes through. This pdf is "printed" without problems and attached to the email the customer receives.

####### ########## UPDATE: ############# ##########

There are 12 orders in my overview in admin. I now randomly clicked all of them. And tried to print. One of them was working now. But only once. If I try again, it's showing same error as before. -__-

####### ########## UPDATE 2 ############# ##########

Haha, today another invoice from the list is printable. And this time, also more than once. This is driving me crazy...

Answer 1

finally I found the answer to this mystery. Well unfortunately the german language has those absurd letters ä, ö and ü. So I used an ä within a comment in the invoice.php. And that messed up the headers thing. I Don't know why sometimes a print went through but now it's solved.