Grosseschnauzer:
There's a bug in the forum software that often doesn't allow a user in a group to post where that group is supposed to have that ability unless the permission set for that group is also marked for that account.
When the user is switched to another group, that permission set also has to be marked otherwise you will run into Cormack Stark's problem. And the permission set for the former group has to be unmarked.
Now I understand it shouldn't have to work that way, but what I just outlined is the only work around that seems to avoid the problem.
This isn't 100% accurate.
If
no custom permissions are set, then the group permissions apply.
If
any custom permissions are set, then
only the custom permissions apply.
For this reason I prefer to use custom permissions
only when they are necessary: when someone has another status beyond the statuses implied by their group membership.