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Chatter Issue: Cannot Create Group or Post in My Developer Edition
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?

Maybe your session expired. Try to login again, and create a group or do something in chatter.
I tried that as well as opening up a new browser. I get error in both Chrome and IE.
Something strange happens with security and the system is not recognizing who you are. If you fix this issue, make best answer or post the solution back here for future reference. Thanks.