Thanks to a conversation with Abhi from India (see this post for the open discussion) I got aware of a bug in the Nagios wrapper-plugin check_win_net_usage.sh.
When the plugin was used and the bug was hit, the output could look like this:
Network OK - check_nt: Could not parse arguments
A bug in the password handling was fixed in todays released version 20121010.
check_win_net_usage is a so-called wrapper plugin which makes use of check_nt and is used to measure the network traffic of a network interface on a Windows host. It combines both incoming and outgoing traffic in the same output which can then be used to create graphs with both values.