Update of the problem:
It appears that it is not the specific usernames. The problem seems to exist in the threads themselves. Some threads, it seems, are not allowing replies, while others are. I can't find a simularity between the threads that are affected. But for example, if you try to post in this thread:
http://www.net-benefits.net/showthre...&threadid=3187
You can't.
But you can post in this one no problem:
http://www.net-benefits.net/showthre...&threadid=3135
Jed