(triple post now tidied away
)
* If you're posting to a thread you've already made posts to, this error can be ignored.
After posting to a thread, the forum software checks to see if it is already being watched and if not (as in the first time you've posted to a thread), the "Watch this topic for replies" status will be set automatically.
But if you've already made a post, this will have already been set - so all the error means is that the currently active watch status couldn't be retrieved - but it's not changed, so the watch remains active anyway.
* If you're posting to a thread for the first time, things get a little more involved because now, no automatic watch would be set up at all.
One way of setting it manually, after posting, is to use the browser "back" button to get to the point immediately before you started writing the post. Be aware that this might take several clicks back through all of the edits you might have made to the post!
Now, click the browser's "page refresh/reload" button and you'll see your post was made successfully.
To set the watch status manually, go to the first page of the thread and click the "Watch this topic for replies" link at the foot of the page. (it only works from the first page)
The error causes people to think that a post has failed because of the sequence in which the software does things. Normally, the post would be made, the watch status checked or set as necessary, and finally the "post successful" message is displayed.
Since the watch status couldn't be checked, despite the post actually succeeding before this step - the confirmation message isn't displayed because the whole sequence wasn't completed.
In no case does it mean that your post has failed, so there is no reason to repost.
If your browser supports it - when making a post, instead of clicking on "Post Reply" or "Quote" as usual with the left mouse button, use the right button and select "Open link in new tab" from the drop-down menu (the wording varies between browsers).
This has the dual advantage of leaving the post to which you're responding nicely visible in one window, with a new window ready to type your response into.
Now, when posting from this new window - the status of the thread and your new post to it is easily checked by simply refreshing the first window.
No back button required.
Another tip: copy/paste any post you really don't want to retype in the event of a posting failure into Notepad (for Windows users) - or some other plain text editor, and save it as you go.
No lost post, no tears. 
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Giraffe: a ruminant with a view.