Windows created have weird dimensions on Debian with Fluxbox

Hi list,

Windows created in the program "Supercollider" (Debian package
supercollider, binary scide) are displayed only with their title bar, and this
bar has very large horizontal dimensions, reaching beyond my desktop
borders. The wmctl utility reports the dimensions 65535x65535 for this
window:
 $ wmctl -l -G 
 [...]
 0x0100001d  0 60   100  65535 65535 zoot localhost Node Tree
This happens to other windows of the same program too.

I can only get to see this window's contents by maximizing it,
which does not allow me to work at this programs main window at the same time. 

This does not happen with, say, openbox. There the window has the
correct geometry of 400x400 there.
 $ wmctl -l -G 
 [...]
 0x00a00025  0 5    40   400  400  zoot localhost Node Tree

I can resize the weird window in fluxbox with the same wmctl command
 $ wmctrl -r localhost -e 0,0,0,400,400
which makes it display normally. However, if I close and open it again
it is back at 65535x65535.

I'd be happy to learn about all possible sources for this problem,
thank you so much!
Peter