This is caused when a solution is inserted into the environment that both a) had a different rollup applied and b) contained the user roles as well. The user privileges/roles since UR 5 now have attributes related to activity feeds for Post, Reads etc.
The key to finding this was to trace the error, and then query the GUID provided in the trace file in the permissions table using the permissions id. The hint was the prvPOST was missing.
To fix, apply UR 6 to the effected environment will add the required privileges back into the mix and resolve your issue.