Have a look at this discussion:
http://moodle.org/mod/forum/discuss.php?d=111468
and my suggested workaround at the bottom of the discussion.
This must be a bug. It should not be necessary to force teachers to access the complicated roles admin just to make an activity work in a normal way.
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Ši diskusija buvo užrakinta, todėl jūs negalite pateikti atsakymus joje.