Munin Service.pm line 263 error

Question

On a Munin Node i added the following plugin

https://github.com/ifad/munin-contrib/blob/master/plugins/network/avm-fritzbox-wan-traffic

and got a error running munin-run

$ munin-run avm-fritzbox-wan-traffic
**Can't exec "/etc/munin/plugins/avm-fritzbox-wan-traffic": No such file or directory at /usr/share/perl5/Munin/Node/Service.pm line 263.
# FATAL: Failed to exec.**

the line is

259     my @command = grep defined, _service_command($self->{servicedir}, $service, $arg);
260     print STDERR "# About to run '", join (' ', @command), "'\n"
261         if $config->{DEBUG};
262
263     exec @command;
264 }

is there a module missing for running this exec command?

Here is how I installed

http://www.wombat.ch/index.php?option=com_content&view=article&id=9484:munin-node-avm-fritzbox-monitoren&catid=64:munin&Itemid=101

and other Munin-plugins works fine with Graphs.