Added in January of 2018: This thread is now ancient history. The forum staff never had any control over this part of the software. We only know that for a couple of years (roughly 2015-2016), posts containing certain words could not be posted, but the prohibition had stopped by early 2017 (though it took us a while to notice). So, short version: Never mind!
We have recently discovered that the forum software is rejecting posts which contain certain product names -- popular pharmaceuticals, expensive watches, and other items (added: or services) that are apparently suspected of indicating spam activity. Unfortunately, this is nothing that the forum administrators have any control over -- it's built into the software.
When such words are used, the software doesn't even come right out and say, "You can't use that word." It merely generates a generic-looking error message:
You don't have permission to access /forum/index.php on this server.
Additionally, a 404 Not Found error* was encountered while trying to use an ErrorDocument to handle the request.
* now corrected to "a 403 Forbidden Error"
If you suspect your post is being rejected on these grounds, try deleting the product name and see if that helps. (Hint: Misspelling the name may work -- e.g., using the numeral 1 in place of the letter i or zero in place of the letter O or putting spaces between letters. Or you may be able to mention the type of product rather than the name.)
If you have questions about this matter, the staff will be glad to answer as best we can. And if you have additional suggestions for dealing with it, please post 'em here!
Here's a list of the "forbidden" words and phrases that people have stumbled over so far (intentionally misspelt in order to circumvent the filter):
A n a l
Degree 1n radiology
dr0p to the table before