Weird client messenger problem - Won't send word 'ok'!?

OK,

This is a really weird error. Everyone in our organisation has windows
messenger configured, and they are all connected to our LCS2005 server.
Everyone can chat to everyone fine, until someone responds with 'ok' to
someone else. Once they send ok, they will get a "The following message
could not be delivered to all recipients:" error, after which they won't be
able to send any more messages until they sign out then back into messenger.
Is there a filter or something on certain words?

Here is a sample of a conversation I just tested:

Ben says:
test
Liz says:
Got it
Ben says:
k.o
Liz says:
Got it
Ben says:
ko
Liz says:
Got it
Ben says:
ok

The following message could not be delivered to all recipients:
ok

Ben says:
test

The following message could not be delivered to all recipients:
test

Ben says:
now it's dead

The following message could not be delivered to all recipients:
now it's dead