There are not currently any low-cost airline routes between Rostock-Laage and Stockholm (Skavsta) with Ryanair.
Instead you could try browsing budget flights between Rostock-Laage and Stockholm (Skavsta) with all airlines.
Another option would be to try browsing all low-cost airline routes to Stockholm (Skavsta) or all budget routes from Rostock-Laage.
In the meantime, this error has been logged.