This had me scratching my head in frustration for a few days as there was no instruction anywhere! All I could do was reply to existing posts using "Quick Reply" - the advanced options or "New posts" did not work - I couldn't even send a message to admin - ALL I kept getting was a blank white screen each time I submitted the message!
I'm not sure at which point I fixed it (following some member's suggestions) as the screen I had set up to repeatedly try a test posting needed clearing and re-starting to clear the problem before I was able to post. But I can post now thankfully!
If this is affecting you try the following (this is all I did):
1/ Fill in your profile with DOB and location and as much as you can for good measure.
2/ Click away the PINK banner messages informing you about fighting spam (cross in corner)
3/ Try posting in the "New Member Introductions & Site Help" first
4/ Reply to over 10 messages using "Quick Reply"
Note that 3/ and 4/ may be red herrings so try 1/ + 2/ first. Time may also be a factor i.e. time since joining. Hopefully an administrator will add some clear instruction about this. Apparently there was a spam problem recently so some changes were made and that's the reason for the confusion.