I have an external object (OpenSearch provider) that I have made available via Global Search in my community.   When I am in builder and I preview, it works fine.  When I go to a user and 'log in to community as users' I receive an unhelp error 'Hmmm something is not right'.   

When this user performs a search, the request is never been sent to the OpenSearch provider.  I strongly suspect a permission issue but I think I have check the permissions over and over and they look right to me.   Following the instructions at "Set Up Federated Search in Communities (https://help.salesforce.com/articleView?id=networks_search_federated.htm&type=5)"

I created a permission set called "FederatedSearchSet". In the object settings I found my OpenSearch object and it is available, visible, and enabled and read access has been granted to the object and all its fields.

I have gone to the 'Members' tab of my community administration page and I have added my permission set "FederatedSearchSet".

(Incidentally I have added the following profiles: SysAdmin, Standard User, and the profile that I created when I created the community site. called 'My Community Plus' .

There is a user account I use for testing called 'April'  When I check her details I can see that hey profile is  'My Community Plus'

So what gives?  In builder mode, all searches of the external OpenSearch object work perfectly.  So the OpenSearch server is answering the phone, it is returning data, and that data is displaying in the global search results.  So there is nothing wrong at the provider side.

When I login to the same community as 'April'  the search query is never even sent to the external provider.  Some settting is preventing it.  I am kind of at a loss, any ideas appreciated.