Had installed ZM with bttv cards. Piece of cake...
Today, new challenge: USB Creative Labs OV511 USB webcam.
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The bttv is working.

This is working:
vidcat -d /dev/video0 -p y -s 640x480 > /var/www/htdocs/test.jpg

This is working:
gqcam -v /dev/video0 -d /var/www/htdocs/test.jpg -t JPEG -i YUV

This is OK:
# zmu -d /dev/video0 -q -v

Video Capabilities
Name: OV511+ USB Camera
Type: 513
Can capture
Can capture subareas of the image
Video Channels: 1
Audio Channels: 0
Maximum Width: 640
Maximum Height: 480
Minimum Width: 64
Minimum Height: 48
Window Attributes
X Offset: 0
Y Offset: 0
Width: 640
Height: 480
Picture Attributes
Palette: 10 - YUV420
Colour Depth: 12
Brightness: 35328
Hue: 32768
Colour :30976
Contrast: 13824
Whiteness: 26880
Channel 0 Attributes
Name: Camera
Channel: 0
Flags: 0
Type: 2 - Camera
Format: 0 - PAL
This do not result in any error message:
# zmc -d /dev/video0

At debug lever 4 zmdc.log do nor show any error message.

BUT

This is Slackware 11 and kernel 2.6.18.2.
Could a bad httpd.conf prevent image/stream to work at all ?