I was getting the same error message too.
Clear your cookies and log in again - it should work then. I have provided a screen shot just so you know I am not making it up :)
After a bit more digging it appears that there is an issue with Twitter. You can switch back to the old twitter but you have to use the link that appears in the blue banner at the top of the page.
If you try to use the option in the drop down called
Switch to Old Twitter it won't work.
I think the reason for this is that it executes a HTTP POST with some hidden form values. One of these values is the
authenticity_token. This value is not set for the drop down option. However, looking at the link in the banner, the value is set.
I suspect this is what is causing the problem.
By clearing your cookies your are forcing Twitter to display the banner which has the correct hidden form values set. This is what enables you to view the old twitter.
This issue has now been fixed by Twitter and you can simply switch to the old version of Twitter using the drop down menu option.