I'm currently experiencing an issue in SXA where the log reports an error and no search results are returned.
3764 08:00:38 WARN Results endpoint exception
Exception: System.ArgumentNullException
Message: Value cannot be null.
Parameter name: objectToSwitchTo
Source: Sitecore.Kernel
at Sitecore.Diagnostics.Assert.ArgumentNotNull(Object argument, String argumentName)
at Sitecore.Common.Switcher`2.Enter(TValue objectToSwitchTo)
at Sitecore.XA.Foundation.Search.Services.SearchService.GetQuery(SearchQueryModel searchQueryModel, String& indexName)
at Sitecore.XA.Feature.Search.Controllers.SearchController.GetResults(QueryModel model)
This appears to work fine in both Standalone and ContentManagement roles.