error origination unclear

[Dormilich]

My website system is giving me the following error message, where I don't know where it originates or how I can fix it. I get this error repeatedly... the server log only differs in the IP address.

Warning [2] - Wed, 01 Apr 2009 22:10:21 +0200
 
Cannot modify header information - headers already sent by (output started at ***/main.errorhandler.php:37)
 
  Function: KBL_main_ErrorHandler,
  Class   : ,
  File    : ***/main.errorhandler.php,
  Line    : 48;
#0 [internal function]: KBL_main_ErrorHandler('Content-Type: t...')
#1 ***/class.headers.php(118): header()
#2 ***/class.htmlplus.php(143): Headers->printHeader('Head', Array)
#3 [internal function]: HTMLplus->__call()
#4 ***/main.php(21): HTMLplus->Head()
#5 {main}

• line #3: line 37 refers to a notice output (E_NOTICE or E_USER_NOTICE)
• the notice must be triggered while instantiating the main object (because the header function is called directly after it, before that is only the redirector (which is not triggered with those files) and the script loader)
• the User Agent must be IE or related (an UA that does not understand application/xhtml+xml)

this is what the server log tells me is the page request

65.55.217.32 - - [01/Apr/2009:22:10:22 +0200] "GET /main.php?f=fig HTTP/1.1" 200 17301 "-" "WinHttp" www.kulturbeutel-leipzig.net
65.55.217.32 - - [01/Apr/2009:22:10:22 +0200] "GET /bilder/trailer-figaro.mov HTTP/1.1" 200 66905 "-" "WinHttp" www.kulturbeutel-leipzig.net

anyone knows what UA "WinHttp" is?
the file "trailer-figaro.mov" is not linked any more since 25-03-2009 (but still present)
the file size (line #2) is not the movie's file size (too small)

I could imagine that the page was cached (no request of JS, CSS and pictures)

can anyone shed some light on this?

thanks in advance

[Dormilich]

found a cause:

Error [8] - Thu, 02 Apr 2009 17:24:24 +0200
 
Undefined index:  HTTP_ACCEPT
 
  Function: KBL_main_ErrorHandler,
  Klasse   : ,
  File    : ***/main.errorhandler.php,
  Line    : 48;
#0 ***/class.headers.php(137): KBL_main_ErrorHandler()
#1 ***/class.headers.php(95): Headers->changeMIME()
#2 ***/class.htmlplus.php(139): Headers->__construct('Head', Array)
#3 [internal function]: HTMLplus->__call()
#4 ***/main.php(21): HTMLplus->Head()
#5 {main} 

now the question remains, how do I handle that? sending a 406 (not acceptable) status code? (or maybe 412 or 417)