System Administration - Cannot add header information error

[futurama]

In the system administration I get some errors.

for example:

In the Module List Changing the visibility, I get this error:

Warning: Cannot add header information - headers already sent by (output started at /usr/local/httpd/wi/docs/xtra/dotproject/classes/ui.class.php:503) in /usr/local/httpd/wi/docs/xtra/dotproject/classes/ui.class.php on line 504

I get the same error at:
- configuring the forum
- activate/deactivate some modules
- changing system variables etc.

I figured out, that in the line 503 a function ob_implicit_flush() is used.
After comment out this function, everything works fine

What exactly does this function do?

I have to use an older PHP-Version: 4.1.2

Thanks for the answer