You may need to access your account page, then return to the forums so you can be properly "recognized" as advanced, and be allowed to post. It's a long-standing issue, mostly involving the quality of integration between the forum and the rest of the web site.

I myself have found that after writing a lengthy post and submitting it, the forum has during my writing timed out, giving me the advanced user error, and causing me to lose all my work (which is why I always copy what I say before see more posting, now).