To solve this issue perform the following steps:
- Go to ADMIN >> Pages
- Look for a (hidden) page called "Search Results". if it is not present create it (in root level or child of Home) and set not to display in menu.
- Open Search Results page and validate, that it contains a single instance of Search Results module. if not, add it.
- If it is there delete it, and add it again to the same page.