Hi,
I had the same problem with my blog (geekynomad.com version 2.6.2).
Clearing cookies, cache etc, didn’t solve anything.
Since I still had FTP access I tried the following:
1. via ftp rename “wp-content” > “plugins” to “1.plugins”
2. create new dir “plugins”
3. login to wp (which now worked for me)
4. via ftp remove empty “plugins” created in step 2
5. and rename “1.plugins” back to “plugins”
For me this solved the problem.
Best regards,
Thomas