Cannot log in to WPMU Dashboard plugin

Hello,

I have installed the DEV Dashboard plugin. To activate it I logged in. The page reloads and shows the log in screen again.

The site did get added at: https://premium.wpmudev.org/hub/my-websites/ but I cannot acces the dashboard to install other plugins.

I thought it might have been a hiccup on your server's end, but a few days later it still persists.

I've reinstalled the plugin but nothing changes, the error log is not showing anything.

Do you have any idea what is causing this behaviour?

EDIT:
I have just installed the plugin on an other website. This worked as normal.
But now I notice the site I added in the first place is gone from https://premium.wpmudev.org/hub/my-websites/