Compatibility Issue with SureMembers

Sure. If the problem is that Relevanssi is making the search stop working, you need to disable Relevanssi in that search.

Relevanssi has a filter hook relevanssi_prevent_default_request which controls whether Relevanssi blocks the default WP search or not. You need a function on that hook that checks if the search is a SureMembers admin search and if it is, the function needs to return false.

That’s probably all there is to this, but since SureMembers is a premium product, I don’t know how you can tell when you’re running it. There’s probably some identifying feature in the $query->query_vars array, post type or something like that.