I'd first try just closing all browsers and coming back here. To reset the javascript variables used by this site.

If you still get the error message then you might try clearing your browser's cache.

If you were able to post here then it's a transient error. Make sure you're in the right thread that you mean to post in. Any thread that has 751 posts is in fact closed to further posting. Look at the header on your browser to confirm what thread you're in and scroll all the way down to the bottom to confirm that it has less than 751 posts.