Cannot log in to WPMU Dashboard plugin


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: 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?

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