Make sure this is checked
Enable board URL canonicalization
Thanks I just had a look and mine is not checked
If enabled, the board URL setting will be treated as the canonical installation URL. If visitors access your site through a different URL, they will be redirected to the appropriate canonical URL. When enabled, you must ensure that your board URL setting is correct. If it is not correct, areas outside the admin control panel will not be accessible!
But the last two sentences are a bit scary.
How do we know if our board URL setting is correct ?