Staying on login page after login

When logging in to https://vdrforums.org.uk the login works but I remain on the login page. This appears to be due to the return url in the address bar being to https://vrdforums.org.uk instead of https://vdrforums.org.uk.

If I log in to https://vdrforums.org.uk/wp-admin the return url is https://vdrforums.org.uk/wp-admin and I get redirected correctly to the wp-admin page.

Any idea what might be causing this?

Many thanks

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