Hard to guess…
Which Cockpit version + addons do you use and do you use MongoDB or SQLite?
Can you access the login page and enter your login credentials? Or is that page completely broken, too?
If you have access to the login page - can you try to login and than call the url to Cockpit in the address bar? There is a small chance, that something in the response gone wrong (error message on the top of the json response…), so the login page doesn’t redirect, but the login actually worked.
Do you use an addon or custom code, that uses the events
Did you create a folder with the name “check” in the root of your cockpit installation? In this case, the request to
/check wouldn’t work anymore.