Ok, I found the problem and was able to remove it (for now) but it coasts the 
comment: Max bits for target <ifname>: <outbits> - <inbits> bits/sec

In line 740 of makegraph.pl at the very end:
 GPRINT:kbitsout:MAX:\'Max\\: %.$targets->[$targetindex][20]lf 
$$targets[$targetindex][13]\\g\' GPRINT:percbout:MAX:\' (%.1lf%%)\\l\' 
COMMENT:\'Max bits for target $targets->[$targetindex][0]\: $bwidth_in - 
$bwidth_out bits/sec \\l\' $opt_error\n") || die("criaGraficoBits() Fatal: 
Could not print to filehandle RRDTOOL: $!\n");

correct this to:
 GPRINT:kbitsout:MAX:\'Max\\: %.$targets->[$targetindex][20]lf 
$$targets[$targetindex][13]\\g\' GPRINT:percbout:MAX:\' (%.1lf%%)\\l\' 
$opt_error\n") || die("criaGraficoBits() Fatal: Could not print to filehandle 
RRDTOOL: $!\n");

by removing: 
 COMMENT:\'Max bits for target $targets->[$targetindex][0]\: $bwidth_in - 
$bwidth_out bits/sec \\l\'

After that, the file works just fine :) Have fun :)

If someone knows something about rrd, he might reintegrate the comment in a 
corrected syntax..

Greetings
 Alexander