An editing trainer in the course with capability "moodle/site:accessallgroups" set to ALLOWED is not able to reply to a forum post when the group mode of the forum is set to separate groups. Error "nopostdiscussion"

Looking at the code in mod/forum/post.php, line 146 (Moodle version 1.8.2):

$modcontext = get_context_instance(CONTEXT_MODULE, $forum->id);

This record is not in the mdl_context table since this table does not contain the forum ID in the instanceid field but the forums course module ID; so the correct call should be:

$cm = get_coursemodule_from_instance("forum", $forum->id, $course->id); - which returns the course module object of the forum
$modcontext = get_context_instance(CONTEXT_MODULE, $cm->id);

You will find this in post.php of Moodle 1.9, i.e. in Moodle 1.9 the code seems to be correct.