Ah, a fellow FreeBSD user. :-)
JMX has a field named OpenFileDescriptorCount on the mbean named java.lang:type=OperatingSystem. Java-monitor queries that field because it is one of the typical things you run out of on a busy UNIX machine.
On FreeBSD, the JVM counts the open file descriptors simply by opening the list if file descriptors and counting how many entries there are. However, last time I looked, it used /dev/fd and not /proc. I just checked on FreeBSD 7.3 and that is the case.
You can try to remedy this by using these lines in your /etc/fstab:
proc /proc procfs rw 0 0
fdescfs /dev/fd fdescfs rw 0 0
After that, mount them using "mount /proc; mount /dev/fd". Let me know if that fixes this, please.
What version of FreeBSD are you using and what version of Java, please?
Your problem also exposed a small bug in the Java-monitor probe. It should have caught this error and not dump it into your log file as it did. I have updated the probe to deal with this situation properly.
Can you please download the Java-monitor probe again? That should give you the fixed version and it should allow you to monitor your machine whether or not the file descriptor handling works or not.