I'm getting this error in the logs and I can't edit my posts/add themes or extensions

Topics: ASP.NET 2.0
Jul 10, 2015 at 8:51 PM
Failed to decrypt the FormsAuthentication cookie.: Error occurred during a cryptographic operation.

has anyone seen this before. I have the most updated code in there.
Sat at 10:54 PM
I did see this when updating from a previous version that didnt have the singleblog, multiusers or multiblogs and after switching to multiuser mode in the web.config then clearing the logs I dont see it anymore. If you get this and that does not solve try browsing one page at a time after purging logs to find the issue.