Forbidden error on some pages after changing home

Site http://uplymeparishcouncil.org

This is a weird one. I have just finished testing a new WP site and followed the steps in the Codex to now use the root as the home URL although all the files are actually in a /wordpress folder. (Windows IIS7.5. Please don’t say I should use Linux – it isn’t an option!)

Before I did that (reset home, associated changes to web.config, index.php in root, and resetting permalinks), it all worked fine, and now a handful of pages get HTTP Error 403.14 – Forbidden (see for example Governance in the main menu, or Accounts in the sidebar menu).

I can see the pages in the dashboard – although if I try to save them after quick editing I sometimes get a forbidden error in the backend too. Other times, they are saved but with no title and the permalink reverting from ‘postname’ to the default ‘id’ form. Regular editing seems more foolproof.

The pages work when I revert to the default permalinks or if I change the permalink by adding ‘-1’ etc. to the slug, so I guess that’s where the problem lies – but cannot see what’s different about those 2 pages from the rest. Is there a concept of reserved names for permalinks that is triggered by these innocuous titles?

I can duplicate the content and see the copy page in the front end (the only difference is “-2” added to the permalink by WP), but I am more interested to find out if there is a systematic problem here that I am likely to run into again.