tcl 8.3.3: concurrent http::geturl fail to save complete image

tcl 8.3.3, http package 2.3, Windows XP
I try to save the live image from a Mobotix Webcam via
set channel [open $file w]
http::geturl http://CAMERA-IP/record/current.jpg \
-timeout 2000 \
-channel $channel \
-command [...]

Whenever I start 3 or more simultaneous requests before the first one
completes, I get truncated images in the saved files.
1 or 2 simultaneous requests is ok, 3 or more fail reliably.

When I use the same code on linux, I do not see this problem.
When I use simultaneous 'wget' invocations, I do not see this problem.
When I use the same code in 8.4.x, this does not seem to be a problem.

So the conclusion is that this a problem of TCL8.3.3/http2.3 on windows.

The bug database on sourceforge does not show related bugs for
'geturl', so the question whether anyone has seen this before?

R'
.