With Regards to the error you noticed in your log - "ORA-00904: "WM_CONCAT": invalid identifier"
Did you recently upgrade your Database?
The reason i'm asking is because we faced a similar error. Upon research we realized that , WM_CONCAT is not a documented Oracle function. It was working fine until we upgraded our Database to 11.2.X. Then we saw the same error.
To overcome , a much better option is using the ListAgg function.
Also, check the request type for any URL/Link or Attachment fields having a token name longer than 18 characters. Sounds crazy, but this is a known defect. When trying to include URL or Attachment fields in search results, if the token length > 18 characters... it causes the search to not work.
(But if you exclude those fields from the search results, you would get no error).