The error may be caused by the new system. Try this:
If you get the error, check the URL of the forum. If it looks like this:
You should be OK.
If it looks like this:
That is when most people see the error messages when trying to post or reply. It does happen for some in the other direction, however. Try changing the Forum or f to the other and see what happens (note that Forum is capitalized, f is not)