Ok I had the same problem for a few days now. Today I was able to solve it. It seems to be some weird bug where Microsoft forgot you where ever participating in Insider Preview. Weird stuff.
The steps I took:
1. Head to advanced settings.
2. Disabled the preview builds for the system.
3. Restarted as prompted.
4. In advanced update settings, activated to get Insider Builds and choose your account.
5. Now it will prompt you to enable this account for Insider Program. Follow the link.
6. On the website, sign in and klick "get started".
7. In the settings app, choose your ring (slow or fast).
8. Might have to restart again.
Maybe you don't even have to do the local bit on the machine and enabling the account on the website is sufficient. But haven't testet this.
Hope that helps.