Posting new topic gives Access Denied

I'm using Advanced Forum 6x-2.0 beta1 with ACL 6x-1.4 and Forum Access 6x-1.8 on an existing forum which used to work ok. Now however, if the Add Topic button is clicked, it tries to load the page mysite/node/add/forum/xx but just displays Access Denied. I've checked all the settings and permissions many times but with no success.

One point which has set me thinking is that although Forums are listed in Content Types, they are not listed in the Create Content list. Another factor is that in Taxonomy, if I enable another content type (such as Blogs) in addition to Forum Topics, the new content type can be added to the forum.

I'd be very glad of advice.
Mike