As a part of larger GDPR changes, we are making some changes to our API. Starting 20 Jan 2019, we will no longer support user search through /wiki/rest/api/search.
Along with this you will also notice the following changes (with immediate effect):
CQL input query will no longer accept username for fields like user, creator, mention, watcher and contributor but instead accept AccountID, Full name (depending on user’s profile visibility) or public name
Order of results coming from this API might differ from earlier due to some changes in search logic
With these changes, we have also introduced a new API for user search () that you can start using today. If you have any questions, please contact Atlassian dev support.