I am trying to set my “home location” on the profile setting, using the map and pin function, even typing in the GPS latitude and longitude. When I click save, all it comes back with is “display name already used”. Yes it is…it’s me! I am trying to change my profile. Yes I am logged in, not trying to add a new account.
I changed my location and couldn’t reproduce your issue.
You can file a report at https://github.com/openstreetmap/openstreetmap-website/issues - maybe it depends on the browser.