Hi Jack,
I have set up and used MailPoet 3 already, but had recently updated it.
When I tried to use your direct link I did get to the subscriber section, and the navigation on the backend also showed me the correct page again. I tried on a different site (I had this issue on multiple sites) without the direct link, and it was working again as well. Perhaps there was only a temporary problem, or it had something to do with the time since the last update. In any case, all is working as it should do again.
Thank you for your quick response in any case!
With regards,
Sander Liket
AquaSound