Using Fluxbox-1.3.7:
$ fbsetroot -solid black
X Error of failed request: Bad Match (invalid parameter attributes)
Major opcode of failed request: 70 (XPolyFillRectangle)
Serial number of failed request: 18
Current serial number in output stream: 19
$ xsetroot -solid black
$ echo $?
0
$
I found a patch for bug 1093 that looked about right and tried to apply that. It still seemed to have the same failure.
I tried to install your release-1.4.0, but that failed to compile in Slit.cc, "error: 'Fluxbox' has not been declared..."
Is there anything I can patch with to fix this problem?