Users that are not CMS administrators are unable to use the Workarea search to find groups or users in the folder permissions section. However, these users can manually browse the list of groups and users and make assignments that way with no issue.
The following error appears after selecting user or group from the drop-down, entering a search term and clicking the search icon: Error: The current user does not have permission to carry out this request.
NOTE: The solution pertains only to 9.00 SP2 sites that also have CU15 (or higher) applied. If you are seeing this issue without that CU, leave a comment with your configuration.
Important: Backup your files before making changes.
Change line 469 of \Workarea\controls\permission\selectpermissions.ascx.cs to:
if (long.TryParse(Request.QueryString["id"], out id) && _ContentApi.IsARoleMemberForFolder(Convert.ToInt64(Ektron.Cms.Common.EkEnumeration.CmsRoleIds.AdminFolderUsers),id, UserContext.GetCurrentUser().Id ,false))