Unknown server error when accessing Search api via REST

Question

When trying to access the SharePoint search service via REST, I always get an internal server error with no information about the cause of the problem.
My SharePoint app uses app-only permissions, and includes search permissions as follows:
  <AppPermissionRequests AllowAppOnlyPolicy="true">
    <AppPermissionRequest Scope="http://sharepoint/content/tenant" Right="Write" />
    <AppPermissionRequest Scope="http://sharepoint/social/tenant" Right="Read" />
    <AppPermissionRequest Scope="http://sharepoint/search" Right="QueryAsUserIgnoreAppPrincipal" />
  </AppPermissionRequests>
I see the search permission listed when installing the app.
I know I'm getting a valid bearer token, because all of my other requests work fine. Here is the request I'm making that returns the error:
REQUEST:
$ curl -H "Authorization:Bearer abcABC...f1-iYrm-efhQ" https://<domain-my>.sharepoint.com/_api/search/query
where <domain-my> is the SharePoint site that I'm accessing. I've also tried this request with all combinations of headers, including Accept, Content-Type, odata, and even including the request digest. I've also tried adding query parameters, and using
POST with the query in the message body. However, this is always the response I receive:
RESPONSE:
<?xml version="1.0" encoding="utf-8"?><m:error xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata"><m:code>-1, Microsoft.Office.Server.Search.REST.SearchServiceException</m:code><m:message
xml:lang="en-US">An unknown error occurred.</m:message></m:error>
If I log in to SharePoint I can make a get request to /_api/search/query and it returns a non-error response. What is causing the problem with my REST request? Is there any way to get more information from the server? This is SharePoint online, so I don't have
access to the server.
Thanks!

Answer 1

When using search you must have a user context, it cannot be used with an app only policy. This feature has been submitted to Office User Voice. You can vote for it here:
http://officespdev.uservoice.com/forums/224641-general/suggestions/6069886-make-it-possible-to-use-sharepoint-search-with-app
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