headers already sent message using webdav - what's it mean?

Hi,

I'm running Gallery2.3 on Mandriva 2008.0. I just configured webdav on the server. I then followed instructions to mount the webdav share on my Windows XP box. When I bring up windows explorer, I see the content just fine. When I transfer files from my local drive to the webdav share, I get the following error in mandriva's /var/log/httpd/error_log:

[Sat Nov 15 12:20:09 2008] [error] [client 192.168.0.xxx] PHP Warning: Cannot modify header information - headers already sent in /data/html/www/gallery2/modules/core/classes/GalleryPhpVm.class on line 142

My error settings in PHP are:
log_errors = On
display_errors = Off
error_reporting = E_ALL & ~E_NOTICE & ~E_STRICT

The files appear to have gotten to the target folder, but it returns an error box to explorer when doing the transfer.

Thanks,
Rick