I am on the Validation Rules Trailhead but cannot get passed the challenge as i keep getting
 Challenge Not yet complete... here's what's wrong: 
This challenge has already been completed
I have deleted the validation rule and re-done it but still get the same error message.

I am trying to complete the Visualizing Your Data Unit but when i try to save and run the Dashboard I get the following error 'The page you submitted was invalid for your session. Please try your action again'. 

I have logged out and restarted but i still get the same error.

HELP :(

I first got this error when trying to create a new group in the first Chatter challenge. I noticed I also get it when trying to use Chatter anywhere within my developer edition. Any ideas?