Why is bubbles() creating empty png graphs?

This code below produces empty XXX.png files - if I use plot(), it works 
and if I enter the commands

png(filename=fn)
bubble(positions, do.sqrt=FALSE, main=q)
dev.off()

manually, it works as well.

I am lost - any help appreciated.

The weird thing is that it worked before I made some changes... (I don't 
have a copy of the working version...).

R version:

platform i686-pc-linux-gnu
arch     i686
os       linux-gnu
system   i686, linux-gnu
status
major    2
minor    2.0
year     2005
month    10
day      06
svn rev  35749
language R

Operating system: SuSE 10

Rainer


-----------------

library(gstat)
library(RMySQL)
library(DAAG)

m <- dbDriver("MySQL")
con <- dbConnect(m, group = "renpatch_renosterbos")

tbls <- dbListTables(con)
runs <- 1:5
years <- seq(98, 0, -7)

progress_end <- length(tbls) * length(runs) * length(years)
progress <- 0

for (year in years)
{
fd <- paste("/home/rkrug/Documents/R/plots/Y", year, sep="")
dir.create(fd, showWarnings=FALSE, recursive = TRUE)
for (run in runs)
{
for (tn in tbls)
{
fn <- paste(fd, "/", tn, "_R", run, "_Y", year, ".png", sep="")
progress <- progress + 1
cat(progress, 'of', progress_end, " | ", fn, " | ")
flush.console()
if (!file.exists(fn))
{
q <- paste("select X,Y,RX from", tn, "where (_run=", run, ") AND 
(_Year=", year, ")")
rs <- dbSendQuery(con, q)
positions <- fetch(rs, n = -1)
while (!dbHasCompleted(rs))
{
cat(".")
flush.console()
}
cat(length(positions$X), "points")
png(filename=fn)
bubble(positions, do.sqrt=FALSE, main=q)
#plot(positions)
dev.off()
pause()
cat(" | Done \n")
}
}
}
}