From the mailing list :
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Been playing about with Forums in 5.1.3 and I can't seem to get the Access Rights to a Forum (or topic) via Groups to work as I would expect. Basically if I grant 'Write' (or even 'Manage Everything') to members, applicable users can create new Topics, but cannot reply. I can only get it to work if I specify individual users explicitly.
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a simple clean session.hasPermision should do the job.