Okay, it only worked for me after adding back my .npmrc, replacing the old contents of that file, then once again clearing package-lock.json and re-installing dependancies. Since this was very unclear, I first tried replacing my .npmrc and just setting it to the standard registry, without any better idea what to do, and that ended up working for me:
npm config set registry "https://registry.npmjs.com/"
Not sure what else would also work here, but in any case this finally solved it for me.