When I do a REST API call to a valid Jira REST endpoint (with valid parameters) and using Jira access token, but the user has no access to the project, then Jira cloud properly returns HTTP 401 code, but the HTML page returned is:
<p>Encountered a <code>"null - null "</code> error while loading this page.</p>
I thing the message was more meaningful in the past. Please, make the text more meaningful (again 
).
Note: the REST API request is e.g. GET https://reqview-dev.atlassian.net/rest/api/3/project/MYPROJ?expand=issueTypes%2CissueTypeHierarchy