I use this service every time I create a complex print, the free version of NetFabb only goes so far and often fails to repair some things.
The service is powered by NetFabb but recently Microsoft switched the output format to 3mf, which wont import into free NetFabb (I haven't updated, so sorry if I'm wrong about that).
Anyway, they have kept the old service live and it can be found at this link: https://netfabb.azurewebsites.net/
Not sure how long they will keep it up for however!