You need to be signed into Reddit unfortunately for it to redirect to the Old New UI, otherwise Reddit will force a redirect to the latest version, causing this error, as it conflicts with the addon redirect function.

Edit: It seems that Reddit has now broken, or disabled, new.reddit.com for some users. Other redirect extensions are having a similar issue apparently. I'm not sure if this is something Reddit will fix, or if they are doing it deliberately to force users onto the latest version :/