Will have a go ...
I think that the problem arises because a hidden thread name is added to the visible part of your reply. By default, the hidden threadname is the same as that given to the original post on the thread (e.g. in this thread it is
Trevor; but in the rare problem threads the person posting has used quotation markers around the thread name).
However ... the way quotation markers in a thread name are handled by the webserver doesn't work properly - leading to an empty (null) thread name being added automatically to any attempted reply. This renders the reply invalid and so it cannot be posted.
JimC