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Missing authority means that you are using the posting key to log in, which explains why you can post but not do other things. You need to find the master password, the one you first used when registering on steemit. Saving password in browser is not a good security practice, you won't find anyone with a lot of steem power doing that. Just a heads up

Thanks for the heads up @snowflake I've now changed the password so I'll try change my settings again and fingers crossed it works. I think saving the password in the browser was actually the problem. It obviously updated in the browser even though I didn't complete the change and therefore didn't save it elsewhere. We live and learn :)