Okay, fixed it myself. On the server, go to /factorioexperimental/base/migrations and delete everything that isn't 0.17 related.
Do the same on your own computer /steamapps/factorio/data/base/migrations (otherwise you can't join the server itself)

That fixed it for me. (so not only Nitrado is lazy ...

I've got the same issue at Nitrado and logged a ticket there. Have you managed to fix it?